Bigger drivers = bigger soundstage


Ideally, I don’t like to be aware of a speakers location in a room and like having sound being well centered for tv/movie listening.  I have two listening rooms and the one with larger drivers (older) fills room so much better vs better speakers (newer) with smaller drivers.  Room acoustics favor larger driver speakers.  
Seems dispersion is better.  The larger driver speakers do not image as well but very enjoyable to listen too. 
So now I am upgrading the smaller driver speakers to better and larger driver speakers and it follows a larger speaker cabinet also occurs.

all speakers herein are from same manuf. 
emergingsoul
Hmmm... you don't say. My current speakers are larger cabinets and larger drivers and do image slightly better than the previous somewhat smaller ones. So I guess you're right. But the ones before that were even smaller and had the best most 3D imaging of anything I ever heard. So I guess you're wrong.

But the first ones had dipole tweeters. So just maybe there's more going on than physical size? Food for thought.
 Room acoustics favor larger driver speakers.  


That's a loaded statement, what specifically are you saying? 
"...The larger driver speakers do not image as well..."

Your post obfuscates at minimum.  We have no clue unless you tell what you are running. 
The larger the driver the worse the dispersion. That's basic acoustics. Only way to change that is with acoustics lenses.

Then again you have two different rooms so the discussion has hit its end.
" The larger the driver the worse the dispersion."    

The larger the driver the NARROWER the dispersion.  Narrow dispersion can be better, it depends on the application.  

Duke

Agreed Duke, thank you for clarifying what I wrote.
In terms of size yes, but the question is WHY the larger driver sounds better. "Saturation" is the best term (for me to understand). The larger drivers are quicker to beam in comparison to their smaller counterpart. That part being a higher frequency driver.

EX: 3 way, front fire. TMB arrangement, rear port Box speaker

IF the XOs design accommodate the drivers, properly and the sensitivity of the drivers is CLOSE (very close <5%). The larger driver is gonna out gun the smaller drivers.. It is as simple as that.. This is from the Speaker box as a complete assembly.

I’m not accounting for a room in any way, only from the actual box and it’s components.

I’ve built speakers and learned, "pick your drivers well". What does that mean? Woofers are woofers.. They work well where the test well. Just because a manufacture claims a driver works from 20 hz to 600 hz
doesn’t make it work well within THEIR test parameters (they lie a LOT). It is ALWAYS dependent on the crossover points with the parameters of a PHYSICAL driver size. 6 - 12" drivers have limitations because of physical size, It doesn’t matter how far you move a cone beaming will occur because of physical size.

Upper and LOWER limits. Don't push 5" drivers to 20hz it's not gonna happen.. Don't push 12" drivers to 2500 hz it physically does not work well.

Today a LOT of the newer speaker drivers are pushed beyond what a physical driver of that size can do because of "BEAMING". Yet because of DEEP throw (HE) drivers, we see it all the time. That is where the dispersion effect falls short. I call it saturation depletion, exacerbated by poorly designed, and untreated room surfaces.

In other words, bigger is better if it’s designed correctly. Every step BACKWARDS, from that position in a simple box speaker, diminishes it’s ability to out perform is SMALLER brother (sister). Apples for apples.

That’s just my long winded BUT simple way of saying WHY..

Large two way speakers, don’t get me going....:-)

I can get all teckie if you want LOL

Regards
Driver size has nothing to do with anything though.
Driver size (physically) is the answer to the OP question. The manufacture, SEEMS to hold to the same principal, without knowing the OPs actual speakers...

The physical size of a typical cone / basket speaker is the only physical limitation of the actual driver... (materials may very) AGAIN 5" drivers don’t do 20 hz any more that 12" drivers do 2500 hz well. I don’t care how well it’s manufactured or wave guide added. They (12") are limited physically.. to lower frequencies, just like smaller drivers are limited to higher frequencies..

The exception is multi driver application.. lobing is an issue then..BUT by careful design (length of the array) in comparison to room height, and XO design ALL of that is just that, "designed into the speaker.". A bad design goes a long ways. Just like a good one does...

Correct driver and application ARE the way to better sound..from the "BOX" perspective only... Nothing else.. No rooms, equipment, cables, tweaks, bad hearing, good hearing, or a change of dippers, matter. ONLY the correct application of a given driver..

When I say correct, I mean I check if the driver lives up to the manufacture claims.. maybe 50% of the one I’ve tested do. Seas is one of the best cone drivers I’ve ever used, just a fact for me.. I wish I could afford their drivers in every build.. Just top notch..

Driver manufactures have been trying to design past it for years... It is just a physical fact... THUS.. Planars... Convex Domes, Rings, and Electro Stats...

I’m not just another pretty face, I’m actually kinda smart...ebm (self proclaimed of course). I know, I can build a better speaker enclosure than most of the older designs.. the newer CnC stuff.. Tough to hand craft some of the designs.. Really no need to . It is mostly window dressing anyway...

Aluminum, @Corian and exotics being the exception..$$$$$ I remember a single log (TREE) went for over 1.2 million just for the veneers. 30 years ago.. I though I spent money that day.. 8-900.00 dollars.. LOL I was just blown away.. For a church build of all things..

Regards
My speakers use a 12" driver, have 360 degree dispersion, a wall to wall soundstage, and can be tuned to any room large or small. They are special! Just saying.
The size of the baffle in which the driver is mounted is a big part of this too.  There have been plenty of A'gon threads arguing about wide baffles vs. narrow baffles.
Smaller drivers with similar bass extension have lover sensitivity.
Lover sensitivity drivers have more thermal distortion and compression. 
You can do nothing against laws of physic.
Sure but Mark Levy works miracles WITH the law.
Small drivers and speakers are good for marketing people:
1. Better WAF
2. Cheaper to built !!!
3. Increase sales of expensive super powerful transistor amplifiers.

IMHO the fashion and huge popularity of low sensitive speakers with small drivers is the one of the biggest delusions in audio.
I don’t talk about really small rooms. For small room small driver speakers are OK.
The larger the cone, the harder it is to achieve the required stiffness while keeping mass reasonable. Your large speaker may be louder and more efficient, but what about cone breakup, whizzering where the outside does not move with the inside, etc.  There is no free lunch, not even with large speakers unless you keep their frequency range low.  This is why several small drivers can provide better overall performance than one large driver, especially if you want a more extended range.
Devaliet Phantoms get bass in the 20hz region at 90db with small drivers, I think they are 4 inch? 
This is the journey.  Different gear and rooms and positioning... And.... Achieve different things.  I just set up my first near field desk system and am loving it for what it is. :-)
It really depends on the manufacture. If you go with cheaper speakers like what millercarbon is talking about, then possibly cheaper larger drivers might be better. With Your better speakers, you can get down to the lower 20hz with multiple 8” drivers which also give you a large soundstage. For example: look at your Raidho and Revel reference speakers. IMO, before you look at driver sizes, make sure your room and front end equipment are able to give you what your speakers are designed to do
you have two choices, flavorize and pick a combination of distortions you like = cat chasing tail......

pistonic motion


Post removed 
My previous favorite speakers were Silverline Preludes that had a D’Appolito array with 3.75" woofers and around 90db efficiency...soundstage was superb. Now my 12" woofed Klipsch Heresy pair image beautifully. It’s the design of the speaker that counts, not the driver size.
Audio2design and Duke have it right. The OP is making assumptions that are incorrect. It all depends on the design of the speaker. Wide dispersion does equal more room interaction. The OP notices better imaging with one speaker but prefers listening to the other which is his prerogative. However, buying a speaker with larger drivers will not assure that he gets the same type of sound. 
The size of the baffle in which the driver is mounted is a big part of this too. There have been plenty of A'gon threads arguing about wide baffles vs. narrow baffles.

<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>

It is still all the rage. The problem with narrow usually means deap to get any volume for the bass drivers. That still leave WIDE reflective surfaces behind the front baffle.

Second; in lue of a place to mount a larger bass driver, they mount in the side of the enclosure.. The enclosure becomes unstable, I don't care what you do.. it will move side to side. What is left to do with this narrow baffle. A smaller longer throw, for the bass driver and mount it in the front or rear. The better solution is to remove the bass all together (my position), and move it to a different enclosure.. ALL of it..

The larger cone surface areas are not easy to protect from back waves. The reason for proper phase plug design.. I don't see it in ANY of current manufacturing. Maybe OPs older design had phase plugs, maybe not, BUT it would have made a lot of difference in distortion in the bass region, adding clarity in the upper regions..

The one manufacture that did do it, went out of business in 2012, VMPS. The guy made his own large drivers from the ground up.
Phase plugs were tuned for the driver application, the same way the magnets were.  Look weird, work great...

Cone strength.. WCF
Woven, carbon, fiber, WOVEN.. Not carbon fiber, Woven carbon fiber..
You can actually see through the CONE material..It has HOLES in it.. Shark skin.. goes against ALL you may think.. thousands of pin holes. 

Regards
Multi driver solution share heat between drivers and improves dynamic and decreases distortions. But it works for small derives as well for big drivers. 
If the biggest soundstage possible is the goal, The amp is the key.

So many rabbit holes herein. All worthy of explanation.  So much so hard to follow.

speaker designers will tell you it’s hit or miss when putting it all together.  
A rigid heavy speaker with ample cabinet room behind a driver with a mounting structure that don’t resonate, is key to transmitting sound.  Larger bass drivers help if they don’t distort, too small ain’t good.  A shoebox size speaker is clearly too small. 
Thus, Magnepan.

Thanks, Jim Winey and family.

Cheers!
The OP headline doesn’t gel all that well with the described findings, it seems. Too many factors are at play determining "bigger soundstage" than mere driver size alone, though better "room fill" is more in line with the use of larger drivers/overall, summed radiation area. Larger radiation area to me generally is about better scaling (which, admittedly, is an aspect of soundstage presentation) and an added sensation of density, impact and ease.

As pointed out by Duke larger drivers have a narrower dispersion pattern, and especially when coupled to horns the sonic marker of this is intensified. As to multiple smaller drivers vs. a single larger one (per channel) a line array of smaller woofers/mids would act as such and limit vertical dispersion, whereas a single, larger driver acts as a point source with narrow dispersion overall. I’ve never much cared for the sound of the multiple smaller driver configuration vs. a single larger one, but whether that comes down to the dispersive nature mostly or other I couldn’t say (I do prefer what emulates a point source presentation over a line source ditto).

Typically the use of smaller drivers involve low sensitivity designs, where the larger driver designs I'm thinking of use high sensitivity units - this is not insignificant either; used within their upper band confines, not least when high-passed down low (which may be a given in the first place as such drivers are usually LF-limited, and thus in the need of subs augmentation), a high eff. large driver sports very low distortion and ample headroom with limited cone movement and inertia build-up. Smaller, low eff. drivers, even in multiples, simply can’t replicate these traits, though they’ll extend higher. Personally though I’d go for a lower cross-over in the ~800-1.2kHz region (with 12-15" units) and have one large driver (or the same in duals) cover the "power region" down ~100Hz, give or take, and avoid a x-over in the upper midrange typically found with smaller drivers. One could also use smaller drivers (8-10") in the same frequency span - say, in a D’Appolito config. - and have very respectable results as well, such as found with the (sadly now defunct) S.P. Technology speakers (by Bob Smith) in conjunction with a waveguide.
I had MAGICO MINI 2s they had a killer soundstage so do Q3s and M2s.
There is so much more to that than just the driver size...
Longer time ago I had a smaller room, smth like 8’4 by 14’, 10’4 ceiling, old apartment building. It could be filled by bookshelves with 6” drivers. Floorstanders with 10” were just too much. I sold the floorstanders to a friend, in his approx 16’ by 17’ by 9’6 from they are much better (even when poorly placed).
my current room is 21’ by 23’, 8’4 ceiling with 7’ by 10’ opening to above + two other openings effectively increasing the volume by some 50%. Used Lowther Fidelios for a time, poor bass with solid state, adequate with 300B SET. That’s 8” + folded rear horn.
Acquired ancient yet immaculate MBL 101c. Base drivers is approx. 9” diameter, probably 13” tall, radially pulsating elongated aluminium ellipsoid. Deepest bass was lacking, but the room was filled so much better than with Lowthers. MBLs need ML 23 to come alive, less power and control is not for them. Wanted cleaner bass, got Gradient 1.5 Helsinki. Got the clean bass (not very much of it though). Overall, almost ‘near field’ listening - truly amazing in the sweet spot, adequate background music in the whole room, yet no freedom of movement like with the MBLs.
Decided to build dipole subs “a la Celestion 6000”, got some BMS 18N862 drivers. Connected them straight to power amp (naked drivers on the floor). 18” drivers, so no heights, on the floor, so terrible early reflections - yet smth. very immediate, coherent, and “right”. Added BSS 966, lifted lows (dipole correction) and heights. Not perfect, but good enough to make me wonder why people insist on boxes, and am I really in a hurry to complete the project as intended. And I have only connected one pair of those BMS, with two per side there will be no shortage of sufficiently deep, yet clean and dry bass. But BMSs do not fill the room. I absolutely need to add omnidirectional speakers from 200 Hz (I.e., above room modes).
I will have two options for omnidirectional, MBLs and German Physiks DDD drivers. When I make ‘enclosures’ for DDDs, I will see if size matters (MBLs have much larger total radiating surface).

Inefficient, thanks

ya know, u raise a point where larger drivers may fill room better where placement offers more flexibilty.  That’s why I feel well designed larger drives offer so much. Yeah!
If I think about your particular case, presuming classic [monopole] bass reflex boxes with cone drivers for low and mid + dome tweeters, smaller drivers can move less air, thus they may sooner run out of their capacity to produce undistorted sound, esp. bass.
But harmonic distortion is only half of the story, bass has to be controlled, and clean or boomy bass does not depend on driver size.
Smaller drivers usually mean narrower baffles and better interference/diffraction and dispersion characteristics, and better directivity (less beaming as frequencies go up).
Directivity has been often ignored in the past, yet it is quite important.
For an example of deliberate choice of narrow directivity look at early “Gradient” models, for example “Gradient 1.3”.
Gradient 1.3’s dipole midrange is larger than it’s boxed woofer. It was done to increase the directivity and avoid early floor reflections.
A the same time its high frequency section is a short line array of four tweeters - again, to achieve broader horizontal plane dispersion and less in the vertical plane.
I did not mention Siegfried Linkwitz, presuming that you are familiar with his work; if not, have a look: http://www.linkwitzlab.com/ALMA'14/Sound_quality.htm
If you go more technical and dry, the measurements here: https://www.princeton.edu/3D3A/Directivity.html
can be quite enlightening.