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.

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.