I actually know the answer to this one
Health-wise, smoking a pack a day is equivalent to being 100 lbs. overweight. Most people would have to gain 100 lbs. to equal the health effects of smoking. It is better for your health to be overweight than to smoke. Smoking kills more people per year than obesity, and even though some public health experts will tell you that obesity is approaching smoking as the leading cause of preventable death in the US, that's only because so many people have quit smoking--and we're still not there yet.
Many people, especially teenage girls, start smoking to lose weight. When you have a cigarette your metabolism speeds up temporarily (like 5-10 minutes). However, it is not enough to make you lose weight, so this plan doesn't really work. Also, when a person quits smoking, the average weight gain is 5-10 lbs, and this usually happens because food starts to taste better. People overestimate the amount of weight they think they will gain if they quit smoking.
Research also supports multiple risky behavior change--meaning, it's okay to try to change more than one thing at a time. The coping strategies you use to avoid picking up a cig are the same strategies you can use to avoid picking up a donut. And the more you practice these strategies, the better you'll get at them.
If anybody is thinking about quitting smoking, I can't recommend pharmacotherapy enough. Use the pills, the patch, the gum, and the inhaler all at once. The tools available today really work.
And weight gain does not HAVE to happen. Many people find that food tastes better once they quit, so fruits and veggies that previously were bland can start tasting better. Also, when you're not smoking, exercise is a LOT easier. Nicotine replacement therapy also attenuates the minor drop in metabolism that comes with quitting smoking. Weight gain is not a sure thing when you quit smoking. Epidemiological research suggests that quitting smoking is the most important thing you can do to extend the number of illness-free years of your life.