Guess you've never seen Motorhead here Jake. And there's plenty of other bands that play exceptionally loudly when they're here.
As Sir HC has previously mentioned, 9 times out of 10 the bands have their own sound engineers, and sometimes they bring in their own PA systems too. The volume of a show is largely determined by the band's volume level coming off the stage, and what their engineer does with it. Also, if a show is very crowded, the audience absorbs soundwaves, whereas if a show is sparsely attended, there are more places for the soundwaves to bounce around, so it sounds louder.