Your metabolism is raised I believe. In theory, you could smoke bud and not eat as a diet type thing...
The natural feeling of "Hunger" is caused by low blood sugar. Smoking Weed lowers your blood sugar, and you get "the munchies."
I don't think anyone really knows, but one of the brain areas with high concentrations of CB1 receptors (to which THC binds) is the hypothalamus. The hypothalamus is involved in appetite control, so there's a good chance that what's going on in there is giving you the munchies.