Banners body has a store of extremely high energy particles. When he transforms, they are converted to much lower energy particles and the energy is converted to mass (e=mc^2). When the hulk goes back to banner, is the same process in reverse: mass is converted to much more highly energetic particles.

OK, so I have the 2009 edition of the Marvel Encyclopedia, which has 8 pages devoted to The Hulk. Apparently he had “every atom of his body exposed to gamma radiation” from a gamma-bomb he was working on. It’s been revealed that Banner also has multiple personality disorder that contributes to his various Hulk forms.

I thought I remember from decades ago that his bulk and healing energy comes and goes from a transdimensional space, but I can’t seem to find that in the article.

However, I really like this StackExchange answer, which posits that he is an energy-processing mutant that taps into the Power Cosmic to fuel his incredible strength. https://scifi.stackexchange.com/a/84410

He doesn’t. He is lower density after transformation. Basically inflated. Also, he doubles as a life raft.

I don’t think The Hulk is real. I’m pretty sure that’s how a lot of things in comic books work. Coudl someone who knows more back me up?

Gamma radiation is probably super high in calories.

The same was the Xenomorph from the Alien franchise grows so quickly - magic.