And you're right, Soy, those are the bodies of "active" girls. You can't maintain a figure like that unless you take care of your body. Having said that, they don't really have "athlete" bodies, they're more "porn star" bodies. I mean, I'm sure porn stars work out regularly to keep their shape, but they're certainly not going to pass as battle-hardened warriors.
Not that there's anything wrong with fictional fantasy characters having sexually appealing bodies, but let's not go kidding ourselves here that it's somehow "realistic". Fighting bad guys doesn't give you plump, juicy tits.