Part of the enjoyment smokers get from smoking, (OK, this is going to sound gay... but stay with me), is the pleasure they get from having something in their mouth. It's called an oral fixation.
When they quit smoking, they still have this desire to put something in their mouth... so they eat more food to replace putting a cigarette in their mouth.
If you can replace the oral stimulation of a cigarette, with something like gum, a toothpick, hard candy, chewing on a pen... who cares... just replace it with something that isn't a lung-dart or something that will make you fat.
Anyway... that's my 2cents...
