Steel Sponge

Why this trope exists

The simplest explanation is that the woman is simply bigger than the other female characters, or simply older and thus more fully grown and developed, or both at once — unlike a flat-chested woman, who is more likely to come across as small, young and more delicate. In American Superhero comics, this is often touted as the reason the Most Common Superpower exists—to establish the difference between younger girls and older women (however, as the trope became more common, there has been less and less distinction between the body types.) In any case, her greater general size leads to both larger breasts as well as greater leverage and muscle mass (not that you'll often see much physical evidence of the latter—compare the biceps on the ladies in the trope image, for instance). If all of the female characters are physically active and she is the oldest, it can also be assumed she's been actively building strength and learning how to use it longer than her juniors have.