The answer to your question is probably rooted in opinion because people disagree over this topic. However, some facts are known. Alot of people that smoke have whats called an oral fixation. This is a psychological component to smoking. When these people quit smoking, they need to replace the oral fixation lost from smoking, many tend to overeat because they are no longer putting a cigarette to their mouth. They compensate for this loss by eating more (more than they used to when they were smoking). Due to increased caloric intake, they tend to gain weight.
Some people that smoke tend to eat less because in some individuals nicotine found in cigarettes acts as an appetite suppressant. These same people report that smoking helps them to stay trim and is one reason they continue to smoke.
It is very common for people to gain weight when they quit smoking. The nicotine that is present in cigarettes acts as an appetite suppressant and can also speed up metabolism. This is one of the reasons that people may not gain weight when they smoke, but then gain weight if they quit.
In addition, smoking numbs smell and taste. After a person quits smoking, their senses of smell and taste return to normal and food may become more pleasurable.
Also, many people who smoke have an oral fixation on the cigarettes so when they quit smoking they feel the need to have something by their mouths, such as food.
Technically, the habit of smoking does not affect weight. However, people who quit smoking discover that food smells better and tastes better, so they eat more. The smell of smoke and the taste of the nicotine dulls the sense of smell and taste. Many people who smoke do so at social times when the lack of a cigarette could be replaced with eating or drinking high calorie foods. People who smoke to quiet their nerves may turn to food as a comfort source once smoking has been terminated. Those who gain weight do so for more of a psychological reason, instead of a physical reason.
One of the side effects of nicotine in the body is that it can suppress appetite. Like any depressant, nicotine, particularly if somone smokes heavily, can make you just feel less hungry and feel hungry less often. So if someone quits smoking, there can be a very large increase in appetite.
The problem is that this increase in appetite comes at a particularly stressful time for the person trying to quit and so turning to food to answer the craving also becomes an option. Along with a likely decrease in metabolism thanks to the cessation of nicotine use, these can combine to cause a great deal of weight gain. But it can be controlled.