Whether your Superheroine (or Supervillainess) is a mutant, an Amazon Princess, or an alien humanoid, if she's female, she is straining the bonds of gravity... but not in the flying sort of way.
The most common of metahuman attributes seems to range from a D-cup bra size upward for a female character just past the onset of puberty (a time when many comic-book characters start to manifest superpowers). The breasts of these characters are not only large, but remarkably self-supporting and perky for their size.
Note that this also applies to Trained Supernormals as well as to characters who have true superpowers.
In the comics, this may fluctuate, depending on which artist is drawing which title at any given time. Generally, this phenomena is caused by the fact that most comic book artists are men, and men tend to like... well... you know.
This is often considered a female variant of the Heroic Build. It is often combined with a Cleavage Window. See Stripperific for costumes that ridiculously sexualize the female form. For the male variant, see Top Heavy Guys.