Why should some speakers need a big room?

It's often said that some speakers need a big room to perform well. Couldn't you just turn down the volume? And if the speakers don't sound nice at low levels, isn't this a fault with the speakers?
(And I never heard any say a pair of headphones needed a big head...)
An obvious response is that some speakers might not fit in the space.

For instance, imagine a pair of Sound-Lab ultimates in a room 10 wide, 12 long with 8 foot ceilings.

The speaker is 38 inches wide, and allowing only 8 inches from the side walls (which is not enough), the distance between them would be only 28 inches.

With spikes, the speaker would be within inches of the ceiling too. Not great for sound or looks, regardless of volume played.
You haven't heard a great large Apogee sound until you hear a properly driven one in a large room. I would think that applies to all large dipoles.
How the room interacts with a speaker defines that particular speakers sound. Imagine playing your stereo outdoors in the middle of a public park versus in an area with boundaries (i.e. living room walls), the speakers will not sound the same in these two scenarios. Remeber, a loudspeaker enclosure not only radiates sound forward, but in all directions that is redirected to your listening spot at various times. These delays cause all sorts of effects, both negative and positive. Headphones are immune to this as there are no boundaries to your ears -- a major reason headphones cannot present a center image

It is not until your system has synergy with the room it occupies that you have uncovered the full potential of any speaker system, be it $100 or $100,000, small speaker or large.
Would you put a grand piano is a bedroom?

This won't be quick or easy if its to be explained correctly.
Let's start with some basics. The Sound Labs Albert refers to are absoutely wonderful speakers, which create sound by vibrating an extremely thin sheet of Mylar, or at least a Mylar like material, a very short distance, by using an electrostatic charge. Because the excursion of the mylar is very minimal, it must move a large portion of the air to create life like 'volumes'--this is one reason why full range electrostatics are large--one moves a small amount of air a great distance, or a great amount of air a small distance.
The stated advantage of the Sound Lab is it's control of this almost infinitessimal weight of the Mylar (or Mylar like material). The SL's are very low in distortion, as the Mylar is, because of this lack of mass, relatively easy to keep in check.
Because the radiating surface of the Sound Lab is so large, it is generally thought to be impractical to have this large of a radiator in a small space. Why?
All rooms, depending on their shape,that is proportions of length to height to width, have resonant 'modes'and "standing waves" that is frequencies,(pitches) which can be disproportionately excited causing, for want of a better explanation, an over reaction to the amount of energy put into the room at a given frequency or frequencies. The rooms also have the liklihood of cancellations of (calculable but not for the typical, "I want music in this room person"). So the result can be a very uneven (frequency anomolies) playback from a very good (tonally correct) speaker if the room can't handle the amount of energy put into it by the loudspeaker.
Here's a cut from a 'resonant mode text':
"The modes of vibration associated with resonance in extended objects ...have characteristic patterns called standing waves. These standing wave modes arise from the combination of reflection and interference such that the reflected waves interfere constructively with the incident waves. An important part of the condition for this constructive interference...is the fact that the waves change phase upon reflection from a fixed end. Under these conditions, the medium appears to vibrate in segments or regions and the fact that these vibrations are made up of traveling waves is not apparent - hence the term "standing wave".

With this in mind, one can increase the liklihood of room excitations being so great as to 'obscure' the basic sound output of the loudspeaker.
Dynamic loudspeakers which usually, (usually) fire out the front--but they have what is known as an omnipolar output. Which is to say, sound coming away from the cabinet in a full 360 degree fashion. The amount of output is determined by how 'active' the cabinet happens to be. Since many of today's dynamic speakers are made of MDF (medium density fiber board) with wooden supports internally, they resonate, giving off these sounds in all directions. How much they give off, is dependant on the amount of mass, (call it weight)that the speaker has to absorb the energy, AND how stiff the cabinet is. The more 'active' the cabinet, the more energy, (additional to the front firing drivers)that the speaker puts into the room. The larger that 'picture' the more room excitations you will typically end up with.
As a designer, one has a choice to make--do I try to 'kill' the cabinet sound by using something almost completely non resonant--or do I try to use that output in some way, and try to make it less of a problem?
It seems that more designers over the past few years have begun to radius the sides of the cabinets. This seems to have the advantage of loading the energy room in such a way as to make that omnipolar energy somewhat less of a problem. It also helps within the cabinet by helping to lessen cancellations, standing waves, (just like those that exist within the room).
After all that, we can now conclude that the amount of energy that a speaker puts into a room, can simply be too much for the room to 'handle' or at least too much to allow for acceptable sound.
Clear as mud, no?
Hope this makes some sense.
If you have a good dealer, do some listening in your room and try to get a feel for what does work in your space.

Good luck.
A large speaker system generally has large drivers with some space between them. The sound from the different drivers (at different frequencies) must blend, and the greater the distance between drivers the greater the distance must be between the speaker system and the listener.
Eldartford has part of it, the other part is amount of air moved by a certain area of driver. For low frequency if the room is too small then the driver will never be operating at it's optimum in terms of excursion and inertia at reasonable volume levels. The converse problem is also true if you try to use too small of a speaker in a large room and try to get realistic volume levels particularly in the low frequencies. The driver has to beyond it's typical limits and just can't fill the volume due to the small area moving air. All of the above is for dynamic speakers--planars have a different set of variables which I haven't really mentioned but many are in previous posts.
Thanks everyone for all the comprehensive & fascinating answers (especially lrsky)!
There sure are some smart people out there.
By the way, I did own Appogee Stages a decade or so ago, and thought there were fine nearfield, but I was fearful of moving on to Maggies or Soundlabs because of the size of my room and always wondered how they would have performed. Thanks again.