Why should some speakers need a big room?

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.