It goes without saying that acting requires some sort of transformation. Actors wear masks - both figurative and literal - adopt entirely different personalities and basically play pretend in an exchange with the audience that requires the latter to put aside their familiarity with the talent. It's all one big game of dress up.
But then there's another type of actor transformation as well, where roles require the actors to commit to changing their bodies more fundamentally and more dramatically over periods of months ahead of filming. Invariably, the best ones are rewarded with Awards (at least partly amplified by their commitment to method), but some cases fly under the radar if they don't involve dramatic weight-loss.
Given that superhero movies tend to require a certain commitment to genetic superiority, the actors chosen to pull on the always-unflattering spandex costumes have to look superhuman. And that means they tend to have to build lots of muscle, cut all fat and generally trasnform themselves into an image closer to the heroes and villains who jump from the comic book page.