Why and how do cigarettes keep you thinner?

Cigarettes are addicting because of all the chemicals that are used in their curing (I find that ironic, curing?) and manufacturing process. Because of this factor, you crave the cigarette, not food.

Psychologically and physiologically, smoking gives you something to do with your hands and mouth other than food. Because of this ingrained 'hand to mouth' action, people tend to eat more when they stop smoking. It is an ingrained habitual physical movement almost as important to the feelings of comfort as breathing.

The nicotine and other chemicals mask the feeling of hunger, so when you get hungry, you don't really realize it and reach for a cigarette instead. Hope this helps.

The drug in cigarettes, a stimulant, increases your metabolism.. So that is one reason some people gain weight when they quit smoking.