It's because huge super muscular women are ugly. -sage nod-

Okay srsly what the hell. They'd already have to make three different costume bits for any costumes that can be worn by all three body shapes, whats's so terrible about adding a fourth?!

(Also, the mean little voice in my head is saying 'it's probably because the heterosexual males with delusions of superiority in the audience would be frightened off by the idea of women who look like they could kick the asses of said males, and you can't lose those subscribers' money!')