Not everyone gains weight after quitting. Some people do, but usually only five or six pounds - and they usually lose it as soon as their bodies adjust to their natural state (a non-smoker).
Two things seem to cause this weight gain while quitting. First, your body metabolism slows down when you quit, so you don't need as much food as when you were using tobacco. Second, you may use food to replace your habit and end up eating a bit more than before.
You do have control over both of these causes. First, if you increase your physical activity, you will burn more calories, even if your metabolism slows down. Second, you can substitute low-fat snacks for cigarettes and get just as much pleasure without the harmful effects of tobacco.
What healthy snacks that you have eaten or are eating during your quit?