Pretty much what Chip said.

I can't even count all the applications I get from insanely hot (in the face) 18-23yo guys who I can not accept because they are too skinny for us. They're more twink than college guy.

And actually, we make it a point to use the term "College Men", as opposed to "College Boys", because we feel that better encompasses not just the age range being 18-23 (rare occasion up to 25... which I guess you could say would be a college man attending a huge state school who can't get in to enough required classes each term so takes a couple more years to graduate? hehehe) but also the body type.

Of course, twink and college guy sites are dealing with guys that fall within the same age range, pretty much. Though the twink sites often contain themselves to 18-21 or so. So it's really body type that distinguishes the genres.