I agree with tswum that callipygic is not the same as steatopygic, and is not necessarily a negative word to use for someone. When I studied History of Art I learnt that "steatopygia" was one of the features of the oddly-named prehistoric Venuses (i.e. Willendorf Venus). These statuettes markedly emphasise those body parts which are linked to reproductive ability: the breasts, buttocks, pubic area and so on. In other words, these bits are disproportionately fat.

I have only ever heard "callipygic" in a medical context, though I have read it in fiction. It seemed to carry no negative connotations at all, and simply refer to a shapely body.