Yeah, it's the body language, because I actually like that PG is muscular(particularly the way Jiminez drew her, which was full-on bruiser) but is still attractive. And no, it's nothing to do with the breasts, dammit.
I think too that we're supposed to be dazzled by her sexiness in public that is making Brainy uncomfortable, and I'm not seein' it. If it weren't part of the point, I'd just say, "Well, some women don't care how they walk in a bikini," but it's at least a little important to the scene in a way you shouldn't have to ponder. And for Chrissakes, it's Hughes.