I think the change in metabolic rate idea has real merit. Of course sometimes people do "self-medicate" with food too, but we are pretty conscious of what we eat here, and I believe anyone who says they didn't up their carbs or calories, yet still gained.

Dr. Schwarzbein says in her book that sometimes the body gains because it's trying to heal itself. So maybe that's another factor in a smoker's weight gain? Sometimes we might need to gain a little to heal.

Anyway as was said above, I think 5, 10 or even 20lbs of extra body weight is way less risky for your health than smoking.