Quite right.  If you start with a big room, then almost anything can be achieved, assuming that it's not a cube and every surface is covered with marble!  If you start with inadequate volume, then there is very little that can be done to make a really good listening room, and it's too easy to overcook absorption.  the downside of a big room is, as has been pointed out, the power required to get a decent listening level. 

Cheers

Shuggie