What makes a speaker too big for a given room?

big speaker vs small speaker

less imaging performance, slower bass, less life and poor low volume performance due to larger drivers

Larger loudspeakers do not have in general slower bass,less life or poor low volume level performance and if one did it wasn't properly designed. For larger loudspeakers actually have less weakness than smaller designs which are total design compromised. Overly small drivers, tweeter covering to much range limited SPL, higher thermo compression,no low frequency and if a attempt is made at reproducing lower frequency you reduce SPL and raise distortion even more. Sure everything has its place and many need or want smaller loudspeakers but to think that the laws of physics does not apply to smaller designs shows a misunderstanding of how loudspeakers function.

Good question.

Drivers are too far apart to sound coherent in close proximity.

Also bass can be too much sometimes and muddy up the sound even
further.

Also most speakers do imaging and soundstage better if away from walls
and this can practically be harder to accomplish with large speakers in a
small room.