I don't get the point of this. I have had anywhere from 900 to 1300 students in my classes.
When I book lecture theatres for my subjects, I ask for something of size >= X + some allowance for the fact that enrolments may blow out, since UNSW is pathetic at predicting how many students I'll have - even at the beginning of week 1 !!!! Also, it's difficult to get rooms at certain times, so by construction my room sizes will be a lot bigger than my class sizes.
Small subject, in contrast, have it much easier to find rooms. So I don't see how room utilisation can possibly be a measure of popularity - unless of course you compare subjects in similar circumstances, i.e. unpredictable numbers, large size, etc.
RobertC (LiC, ACCT1501)