According to the comics, Hulk gets his mass from another dimension. So far, so good. However, this does not explain where the extra body tissue comes from. Energy has specific forms: heat, motion, friction, etc. If you put that energy into a system, such as into a human body, then that doesn´t change the body composition. Additional body mass needs a cell proliferation (and that can not come from the outside).

Assuming that the supplied energy would increase the body's cells instead of causing more mass. Then the problem of the additional mass would be solved. The cells would just grow (and of course also the elementary particles).

Would that be an acceptable explanation?

@zardu:

The number of cells Bruce Banner has doesn't increase, but all his cells just become more massive and more dense.

Hulk is about power, not size.

Oh god i never knew about the dimension thing. I would have just preferred it was a mutation due to cellular growth