He gets the mass from an alternate dimension. First rule of comic-book physics: When all else fails, invoke an alternate dimension :D.
Random note: Anime too.
Simple, the mass gets converted to energy, why do you think that the guy is green most of the time? Radiation, that is why.
A pretty decent synopsis of the comics Hulk can be found here. It’s actually a good resource for a lot of Marvel character info.
As far as the movie Hulk, it’s based on the comic. I’ve read where Ang Lee took liberty with his size, increasing it as his anger and rage grew. Makes sense to me.
For further inquiries about the green goliath, I’d refer to you the Ask The Hulk thread, if the board was running fast enough for me to search for it. 
Hulk was grey and went by the name of “Mr. Fixit” and had a cheufer to drive him around in his limo. At this time he talked like a mafioso.
Thanks to his amazing expanding pants, only She-Hulk knows for sure.
And here I was, thinking that the Thread title was a comics/sexual thing…
Yuck! They’re cousins, you pervo!