What makes speaker's sound big?

Does a speaker need to have many drivers or a large driver area to sound big and fill the room?
I am asking this question because I have a pair of tekton design double impact and would like to replace them with smaller speakers and a pair of subwoofer's to better integrate the bass into my room.
I just borrowed a set of B&W 702S. The are good but the just don't make that floor to ceiling sound that I like.
Maybe I have already answered my own question (: But again I have not heard all the speakers out there.
My room measure 15x19' and the ceiling goes from 7.5 to 12.8'

A bit of harmonics can add up to a big sound.  I don't call it harmonic distortion, but some do.  Mt people like it. B&W aficionados try to hate added harmonics.  I add some to bad recordings by switching out my sub-woofer crossover.  The sound is much bigger when I do that, but loses some pin point accuracy.  Many non-audiophile friends prefer this plump sound.
@mijostyn --

Image density? That is a new one. I suppose if you put the speakers closer together you get a "denser" image, farther apart less dense but larger. Yes, the distance between the speakers can change the image size as long as the listening position stays the same.

Had you experienced larger horn-loaded speaker set-ups you'd be familiar, I can only assume, with how the term 'density' applies to the perceived sound here, although variations with regard to how to explain sound can be a factor. Technically it relates to a larger part of the sound from the point sources here being emitted directly vs what's reflected, in conjunction with prodigious air radiation area. This (i.e.: density of sound) doesn't apply equally with a line source being a line source, from my recollection, even though it shares the traits of less reflected sound and large radiation area.  

However phusis I will repeat this again and having installed and set up numerous very expensive systems, volume has nothing to do with image size. They are two separate issues.

I may have expressed myself incorrectly; by "volume" I meant sheer radiation are. 

A set of dipole line sources going at 90 dB is going to have a much bigger image than any floor standing dynamic speaker going at 90 dB. You can crank that floor stander to 110 dB and it still will not have the image size of the dipoles. As a matter of fact the image size will not change at all.

I'm a fiend for sound that mimics a live event, and the planar and electrostats I've heard - for all their qualities - never left me convinced about a presentation reminiscent of live music, certainly not as much as high efficiency horn-based speakers or even some select, less efficient, direct radiating alternatives. That's line source vs. point source(s) for you, and in this particular context lower vs. high efficiency as well. We can argue all day about "the biggest sound there is" (I don't agree with you here either), but it's all for nothing unless there's some purpose to link image size with what sounds believable overall to both you and I. It seems we're simply not in tune here..

Many would not know this because they have not experienced it. Perfect line source dipoles are rare beasts and hardly ever set up in stores or at shows. It would seem you are talking from instinct and not experience or you would know this for sure. Everyone who has listened to even an imperfect line source knows this. Just ask any Maggie owner.

What exactly am I supposed to know having listened to line source dipoles, a "perfect" one not least? I've only listened to one terminated-at-floor-and-ceiling line source, the Dali Megaline (coupled to the Dali Gravity class A amps), and it was an impressive experience as I recall, now over 20 years ago. Or else I've listened to a variety of Martin Logan's, Acoustat's and Magnepan's over the years, and none of them swayed my into their direction as anything I'd consider to own. 
Your ears have no idea how big the speakers are. It hears sound pressure.
Mikelavigne, where you said....

some good mid bass is also helpful; again the speaker driver surface in the mid bass needs to be matched to the volume of the room so it can propagate. the whole room needs to be energized.

is there a formula  one could follow in order to size the speaker system to the room?
Hi, Martin (OP) 

Much wisdom spread throughout the thread here and elsewhere online. 

My 2 cents... 

For a great condensed reference that touches on all this and much more, you may want to get a copy of Jim Smith's "Get Better Sound" 

Jim is one of the best in the audio / music biz and there are many things within its pages that may help solve your issue or at least improve the quality of what you hear. It is likely to help with other considerations and improvements as well. 

Although his in-person, in-home consultations are pricey, he offers very reasonable 30 minute phone consultations. The book and the chat might be the best investments you can make (or will ever make) in your system and setup.