Boobs of Steel/Analysis

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. 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.
 * Visually, it's a fairly simple design point: the artist wants to indicate and reinforce the notion of physical strength. A large-breasted female tends to look larger overall, and more solidly built. The other route for showing obvious physical strength would be giving the woman a massive bodybuilder physique. As the former is more commonly appealing, and the latter is more commonly intimidating, you can see the obvious choice.
 * There is also the fact that steroid use shrinks female breasts, so putting large breasts on a woman with a bodybuilder-like physique may be to stop the viewer speculating.
 * On the other hand, when they are developed and strong enough, the chest muscles 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. If a woman wants larger breasts and is already working out a lot, this is actually a plausible way to increase breast size by at least two cup sizes.
 * And then there's the Male Gaze aspect.
 * The large-breasts act as compensation for the boobs-of-steel's more masculine qualities, such as participation in combat or other male dominated activities. As if indicating to the viewer that this character is indeed designed to capture your interest and to be viewed as a with-out-a-doubt female.
 * This trope might have come about as a form of visual appeal. Large-chested woman throws punch--what happens to chest
 * The Western perception of a powerful body is that of a fit male, i.e. with a lot of upper body mass. 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.