The most common is that they are simply eating more calories that they used too.  Increased appetite is a frequently reported withdrawal symptom after quitting smoking.  Changes in the types of food that you are eating might contribute to weight gain too.  Also, changes in your body chemistry may have something to do with it.  When you quit smoking your metabolic rate slows down to a healthy level causing your body to burn fewer calories.  Some ex-smokers report missing the feeling of having something in their mouth or oral gratification.  Eating or snacking while going through the withdrawal process may help fill that need while increasing caloric intake.  Finally, food can be a comfort similar to the ways that smoking provided comfort.  People may eat to help them deal with stress, boredom, as a reward, or as part of a social function.  To avoid weight gain, it’s important to know the reasons why you eat.