Answer:
If you're a smoker, you're likely thinner than others. So, if you cut down smoking or quit altogether, you'll pile on the pounds, though no one knows why this is so.....Most likely is because nicotine is an appetite suppressant and seemingly can raise your metabolic rate.....
The US National Center for Health Statistics calculates that smokers kicking the habit have been responsible for a small but significant portion of the US obesity epidemic.
The Center works out that smokers who quit in the previous decade are more likely to be obese than smokers and those who never smoke. Among men, almost 50% of the quitters are obese compared with 37% of non-smokers and 28% of smokers.
Source: http://www.selfgrowth.com/articles/CeceliaYap2.html