I'm probably making a crapload of assumptions about the artist, but I think it's somewhat akin to how you sometimes see Batman or Superman with ridiculously defined and bulging muscles. It's an idealized human form. What I find sad is that at least for the male heroes, idealized means visible strength, but for women it means being hyper-sexualized.
Those boobs would weigh her down and cause considerable back pain, instead of aiding her in any way.