According to a legendary Hulk writer whose name shall not be spoken here, but his initials are a synonym of "cushion," the Hulk basically IS a kind of mad, walking cancer, in the sense that his cells are constantly dividing and building, to, among other things, account for the increase in mass.
I liked Ellis' later explanation--which I think actually can be added to that--regarding how the Hulk's body, if not his mind, learns and adapts and builds.