What makes a speaker too big for a given room?

Poor design. For if the loudspeaker is large but designed to integrate and work in smaller spaces it can. Its many of the smaller designs that are compromised and under sized for proper sound quality. And may require near-field use in small rooms due to extensive Thermo and dynamic compression. Most Subwoofer designs again are compromised in design and one ends up with a large excursion driver in overly small cabinets thus requiring massive power and floor or corner boundary reinforcement to produce lower frequencies. And again Thermo compression rears its head as large power is forced into voice coil. Reducing efficiency, dynamics, transients and adding distortion.

Good points Johnk. How many speaker manufacturers actually build big speakers designed to work well in small rooms?

Everybody seems to want the smallest form factor they can get away with. Unfortunately that leads to the problems you addressed.

It would be nice if manufactures would list what room dimensions are optimal for each model they make. Might make it easier for the consumer to figure out.

My guess would be woofers that create to much bass energy(air movement) and then sounds bloated.

Seems to me that the point of the exercise is to maximize the enjoyment for your budget. If the system is too big physically that it pisses you off then clearly its too big. if the speakers are too big (performance wise) and still sound better then your other options and the money is right then whats the big deal. I recently heard some aerial 20T's in a small room and man that was nice. way better than my more modest units. If I had the cash I'd consider that in a heartbeat. It may be running the aerials in a non ideal state but if you like the sound then who cares. thoughts?

Manufacturers should not only know the dimensions of your listening room, but about the furniture, carpet, drapery, exactly where you plan to put the speakers, and should also conduct a lengthy interview (perhaps in the listening room over at least a couple of days) with you regarding your personal taste, how much you care about the opinions of others, and, of course, examine your finances.

Also, I recently read an essay in some HiFi mag about high quality subwoofing. Although technically maybe there isn't much below 40hz or so that should concern you, the writer was listening to mandolin music with and without his REL sub. It sounded much better WITH the sub even though logic dictates it shouldn't. It seems that a well designed and properly placed sub seemingly "charges" the air in the room with energy of some hard to measure nature. Having done this experiment myself with my trusty old REL Q150e, I experienced the same thing. If I dial out the sub it seems to drain the natural life out of the sound, leaving me in a despondent state of woe not unlike the demeanor often displayed by subless minimonitor users shuffling around their tiny apartments in tattered tweed jackets pausing only to look out the window at a full range world.