Take away the cigarette and they don't know it is time to stop eating. Social situations with food used to be easy as a smoker. When a smoker is done with his food, he can sit and smoke while conversing with others at the table. Without cigarettes, he feels awkward just sitting, so he often orders extra coffee and dessert to last the duration of the conversation. All of these different behaviors add up to one result, extra calories eaten which result in gaining weight.
Weight gain can be extremely dangerous to an ex-smoker.
But this is not because of the strain on the heart. An average ex-smoker would have to gain 75 to 100 pounds to put a strain on her heart equal to the extra risk associated with smoking a pack a day. And then, the extra weight would not cause the lung destruction, cancer risk and many other conditions caused by smoking. The real danger of the extra weight is that many ex-smokers use it as an excuse to go back to smoking. They think that if they smoke again they will automatically lose weight.