A speaker can be too large for a room for any of the reasons already cited. However, specifically as to low bass and room size ("20hz is 20hz"):
Quarter wave cancellation (reflections of omnidirectional low frequency waves - usually off the wall behind the speakers) causes irregular bass response. This cancellation is a function of the distance from the speaker to the nearest large reflective boundary (i.e. the wall mentioned above). Usually - tho not always - this distance is also a function of room size. You might put a speaker 5 feet out in a 20' room, you are less likely to do so in a 12' room.
As to "20hz is 20hz" (subwoofer vs floorstander):
You can place a subwoofer flush to the wall and eliminate a lot of this destructive cancellation at low frequencies. A 20hz wave generated in free space behaves very differently than a 20hz wave generated at the wall. A HIGH QUALITY subwoofer will also typically (as near as I can tell from published data) - tho perhaps not always - exhibit much less low frequency distortion at any given SPL than a full range floorstander rated for similar LF extension.
So, one reason a speaker is too big for a room is that it must, as a practical matter, sit too close to the wall to allow smooth bass response.
Marty
PS This doesn't mean a subwoofer is "better", just that smooth, clean deep deep response is usually easier to achieve with a subwoofer.
Quarter wave cancellation (reflections of omnidirectional low frequency waves - usually off the wall behind the speakers) causes irregular bass response. This cancellation is a function of the distance from the speaker to the nearest large reflective boundary (i.e. the wall mentioned above). Usually - tho not always - this distance is also a function of room size. You might put a speaker 5 feet out in a 20' room, you are less likely to do so in a 12' room.
As to "20hz is 20hz" (subwoofer vs floorstander):
You can place a subwoofer flush to the wall and eliminate a lot of this destructive cancellation at low frequencies. A 20hz wave generated in free space behaves very differently than a 20hz wave generated at the wall. A HIGH QUALITY subwoofer will also typically (as near as I can tell from published data) - tho perhaps not always - exhibit much less low frequency distortion at any given SPL than a full range floorstander rated for similar LF extension.
So, one reason a speaker is too big for a room is that it must, as a practical matter, sit too close to the wall to allow smooth bass response.
Marty
PS This doesn't mean a subwoofer is "better", just that smooth, clean deep deep response is usually easier to achieve with a subwoofer.