Most Helpful Girl
I agree with you as I don't find it humorous either. To answer your question I think a lot of jokes revolve around insecurities and size can definitely be one for guys.
Most Helpful Guy
People will jump to any chance to ridicule others. Especially when it comes to pointing out a person's biggest insecurity. For guys that would be their size. Add in the fact that unlike girls, guys are told to just man up and deal with it so making jokes about them somehow becomes acceptable when if you were to do the same thing onto a woman you'd be considered heartless.
'Cuz guys don't have feelings n shit.