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 woman (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 latterócompare 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.
Visually, it's a fairly simple design point: the artist wants to indicate and reinforce the notion of strength—both strength of body and strength of character. Think on a comic book or manga story with a very blunt and tactless Lad-ette. Now, what size was her chest? In other words, a large-breasted woman tends to look larger overall, and more solidly built. Musculature could fulfill the same role, but larger breasts are far more visually appealing.
Digressing from the previous point, Boobs of Steel are sometimes compensation for masculine qualities, to subvert No Guy Wants an Amazon or Brawn Hilda. A large chest will be added to reaffirm that this is indeed a girl, designed to capture your interest and to be viewed as pretty despite her masculinity. This has the added benefit of conforming to the "top-heavy" stereotype of physical strength. When it comes to female bodies, increasing breast size is an easy way of increasing the mass of the upper torso, without compromising the character's femininity, which adding muscles would easily do.
And finally, if you're looking for a practical explanation, when pectorals are developed and strong enough, they can act as a natural bra, so the breasts look way firmer and that can make them look bigger than they actually are. It's actually possible in Real Life to increase breast volume with large pectoral muscles by at least two cup sizes. Gina Carano would be a perfect example. However, there are limits to how plausible this can be, as the amount of muscle needed to support ginormous breasts like seen in fiction would reduce the amount of fat needed for such breasts to be that size in the first place. This is why many female bodybuilders have no choice but to get breast implants if they want to maintain that kind of feminine hourglass figure.