At least originally, she transformed into a wolf-girl. She didn’t really change her size at all, she just spouted hair all over and grew claws. Nothing that necessarily violated the conservation of mass.
I could be wrong. It has been a while since I read the comic that introduced the New Mutants. But, I remember her doing both. At one point, she changes into a wolf to jump over a fence. She shifts into wolf-girl form to frighten off the guard dogs. Then she shifts back to wolf to jump the inner fence.
Perhaps The Hulk has an absorption mechanism on the soles of his feet (notice he’s always barefoot post-transformation) and sucks up nutrients from the ground like a gamma-powered vacuum cleaner, which are then quickly assimilated into tissue. He is green after all, like a plant. His feet are like roots.
…or: it’s the Hulk; don’t overthink it.
Smart Hulk passes along his knowledge to She-Hulk in the MCU:
Not SF, but Terry Pratchett had the vampire Lance Constable Salicia von Humpeding explain that she couldn’t turn into A bat, but she could turn into a whole flock of bats.
One source of extra matter used in Marvel is a specific other dimension access by “Pym Particles”, the things discovered by Henry Pym aka Giant Man. Iron Man went there once, it was a weird place where everything was all sizes at once; quantum superposition, essentially.
As for the pants if I had to handwave an explanation, his power partially extends to them growing them to (almost) match his body because of a subconscious desire not to end up naked. Which would also explain why they are tough enough not to get blasted off him in his various fights.
The old novel Operation: Chaos does. Lycanthropes can change into animal forms, but only of the mass their human form has; the one tiger lycanthrope encountered is massively obese since he has a tiger’s mass on a human frame.
EDIT: Also, The Madness Season by C.S. Friedman had a sci-fi style vampire who could divert excess mass into hyperspace, and a shapechanging alien who could do the same in the opposite direction as they were native to hyperspace and were choosing how much mass to extrude into normal space.
Still not SF, but Tiffany Aching, in another Pratchett book (A Hat Full of Sky) turns a phony wizard named Brian into a frog. And, since the extra mass has to go somewhere, there was something “like a large, pink, balloon full of water, quite pretty really, wobbling gently against the ceiling.”
She explains: “That’s just the stuff he doesn’t need right now. It’s sort of . . . spare Brian.”
One would think ‘stable molecule’ would be a better term, then.
In the Belgarian novels by David Eddings, extra mass like clothing and carried equipment is shunted off into some kind of extradimensional space by sorcerers when they change shape. Not that they call it that, they themselves have a pretty vague understanding of how it works but it’s pretty obvious that’s what they are talking about.
They are called that because they take on aspects of whatever they are in contact with (like a shapechanger or otherwise transforming superhuman), and can be induced to radically change form.
Alan Moore’s take on the Captain Marvel/Shazam transformation process (young boy turns into super-powered adult) in his Miracleman stories explained that Miracleman’s entire adult body resided in a pocket universe in a state of stasis, and could be summoned by simply saying ‘Kimota’.
A brief glimpse into this (rather creepy) pocket dimension showed that there were numerous other bodies floating there, waiting for various people to call them up.
He’s called the Hulk, not the Bulk. Clearly he gets less dense as he grows, which explains his ability to jump huge distances. He’s essentially just an invulnerable balloon.
In the Gen-V series there’s a young woman who gets smaller by vomiting and bigger by eating. But the amount she expels or eats doesn’t account for the actual mass change.
I’ve been reading Hulk comics for fifty years and this is the first time I can recall hearing this elegant theory. Now some killjoy will post a picture of Hulk standing on a scale.
Gray hulk also appears to skip leg day.
If he can throw a car very far, then his own density is unrelated.
If he was less dense then his punches wouldn’t have much force. It’s like being hit by a very fast sponge ball - not much impact. To get that strike to the point where it starts to do significant damage, he’d have to be so insanely strong that despite the near masslessness of his fists, he still imparts significant damage.
At that point, you don’t really need low density to explain how he can jump so far. He’s just so massively strong.
It’s been demonstrated dozens of times that the Thing’s strength is nearly on par with the Hulk’s but you never see him leaping vast distances. He needs Mr. Fantastic to slingshot him.
How far can he throw a car?
The Hulk? Depends entirely on how mad he is, and who’s writing him that day. I would not be at all surprised if he could put a car into orbit.
As far as the writers need him to.
The FF fought and defeated an alien because of this. He came to earth to feed on energy. He fed and got bigger. Reed measured the alien’s footprints. He concluded that the alien was getting bigger and also less dense. Reed aimed a laser or something at the alien. The alien became so huge and of such low density, he effectively ceased.