Pectin is a biochemical your body creates to regulate diet and weight, most people who aren't smoking have low pectin production. A low production means the cannabinoids from weed can connect with the cb1 receptors (instead of the pectin) in your brain which induces hunger/buzz. A high amount of THC (seasoned smokers) in the system actually stimulates the pectin production and as the levels rise the less hungry you get. Please don't quote me on this, I read it at lunch after celebrating 420, but this is what I remember.
This might be why, after you've been smoking for a while, you stop getting the munchies and its harder for you to get as high as you used to but then after taking a break you get a great buzz and the return of the munchies.
-V