Heroes nowadays

Aquaman?

Even Jason Momoa

Excellent observation. I won’t get into superhero physiology as there aren’t too many hard “cannon” in depth lay outs of how things like protein synthesis work, or the mechanisms by which a human body can safely perform these feats- let alone all the “mutants” and “aliens” and such and how their biology could be completely different- but there are compelling arguments that a “super human” would either look disfigured and have muscles big enough to limit visibility and basic movement, or be relatively slim if we use some basis of known human biology as a starting point.

I won’t go too deep- but muscles get bigger when we make them perform specific kinds of work under specific circumstances, and our bodies have enough surplus energy and building blocks (like protein and amino acids) to build muscle fibers. Water and glycogen retention can add to muscle volume, and some chemicals like steroids can increase the size of muscle cells instead of the quantity thereby making muscles “bigger.” But all in all- basic biology says that genetics, and then eating a lot (right) plus taxing your muscles= mass. If you can lift a jet liner- the workout routine required to get “buff” would be more than just doing push ups every day.