Like popcorn kernels, his muscle cells inflate when exposed to gamma radiation.
Uh? There have been relatively few times when Wolvie has lost appreciable amounts of mass when injured (and those tend to lead to things like him dying or needing to be rebuilt by the day’s Being Of Immense Power), I always thought he just ate like Obelix anyway.
Cyke’s rays are one of the zillion variations on bioblasts. They’re the boosted-up version of fireflies. Why does Scott light up inside his eyes the whole time while his brother emits bolts from his hands and why this second one is the most common form of bioblasts would be more the question (the “can’t switch it off” for Cyke has been explained by “hit his head”, but that doesn’t explain why he gets eyebeams).
Anger. Pure anger, solidified onto the frame of Bruce Banner. The angrier you make the Hulk, the stronger he gets. Turns out anger is green just like jealousy but Jealousy Man just sits and pouts so he never go his own comic.
Ink, they use ink.
Anger has shiny metallic purple armor (and jealousy has a fiery green gown).
And what color are the Hulk’s pants? I rest my case.
Speaking of that, I always wondered why he hulks out of everything but his pants.
Curious, that.
It’s really very simple
As Professor Einstein told us matter is energy
As Professor Rotten told us “Anger is an energy.”
As a side effect of Gamma bombardment, Doctor Banner’s rage is transformed into mass and coalesces around his cells. The angrier he gets, the more energy he manifests and therefore the more mass he acquires.
If you don’t believe me, conduct the experiment yourself and report your own findings.
As I recall, the standard line for characters who grow in mass is that the mass comes from “an unknown, perhaps extradimensional source”.
I wanted to write a miniseries called “Spectrum” in which a Kree seeks to endear himself to Galactus by rigging up a link to the extradimensional mass/energy source and thus bypassing the need to consume planets for sustenance. He’d gather various Marvel characters who fit on the EM spectrum in some way and whose mass or energy cannot be readily explained - Hulk for gamma, Human Torch for infrared, etc.
It doesn’t work, though. Galactus is laconic about it.
Conclusive proof that good old family values are mightier even then gamma rays.
That doesn’t explain why his pants also hide his thighs, though.
If they were trying to target a female audience, putting the Hulk in a thong may well have been the right choice. But they aren’t.
Oh damn.
Nurse! Some brain bleach here, stat!
Reed Richards’ stretching raises the same issues of added mass.
The Vision sends matter back and forth between dimensions to alter his density.
He is a carbon sink. That explains the green. It is all photosynthesis. 
Starving Artist wrote:
Speaking of that, I always wondered why he hulks out of everything but his pants.
Curious, that.
I don’t know that one, but She-Hulk explained her ever present clothing…
Doesn’t a rubberband keep the same mass when it’s stretched?
Sure, but Richards’ limbs don’t get proportionately thinner as he stretches.
I find Hulk’s extra bulk totally understandable.
It’s like the anger version of horny and his whole body is a penis.
Giantman on the other hand. Pffftttt ridiculous.
And purposely poorly explained, I think.
A lot of Sci-Fi (and comics) violate basic laws of physics; in this case, Conservation of Matter.
The YA series Animorphs* got around it by postulating Z-Space, another dimension where the changelings’ extra bulk would go when they morphed into a mosquito.
I remembered they had a character “having trouble focusing-- I’m getting grossed out by thinking of blobs of my fat drifting around in space, in some other dimension.”
*holds up as solid Sci Fi – and it’s fun!