Smoking provides an oral gratification, so substituting food only makes sense. I'm trying to chew gum. Also, nicotine is an appetite suppressant, so an increased appetite would be natural as well.

A friend of mine was overweight. She decided to quit smoking, and she talked to her doctor about it. He said, "That's great, but whatever you do, don't gain weight!" She said that she knew she was in trouble when her doctor was more concerned about her weight than her smoking. She joined a gym, lost 50 lbs. AND quit smoking!