Its the munch that does it, although some people have a high metabolism so it doesnt really affect them either way. I have a crap metabolism, and I eat loads, so naturally Im a fat bastard.
If you get the munch, instead of heading for the crisps, pizza, chocolate etc, plan to get some food before you smoke. Lately Ive been having massive cravings for food that Ive never even eaten before - yam mainly, and the fruit that when you bite into it, the juice just dribbles all down your chin. Yum yum yum.