the category display is actually an offshoot of the main forum summary, and on the forums ive tested it on has not been compressed either.
check this page:
http://www.ispreview.co.uk/cgi-bin/ubb/ultimatebb.cgi?ubb=forum&f=1&DaysPrune=1000 &submit=Go which is 6k or so
or any topic through the cgi link, which is about 4-5k, whereas saving the source comes out to about 34k.