Wherever men in spandex and capes are found, so too are found tight butts, sculpted chests, and washboard abs. Wherever women in spandex can be found, there are large breasts, slim waists, curvy hips, and shapely legs. It could almost be called a secondary superpower in and of itself -- the ability to have a perfect (one might even say over-exaggerated) physique.
Some parts of this may be justified, since many superheroes and villains seem to spend a great deal of their off-time working out religiously. Although one should note that, for women, at least, reducing body fat tends to make the figure a lot less lush than that of the typical superheroine or supervillainess.