Can smoking affect your weight?

Yes. Nicotine is an appetite suppressant, and this has been one of the reasons that women increasingly have taken up smoking. Smoking cessation may result in modest weight gain, which sometimes deters people from trying to quit. Weight gain is usually modest (5-10 lbs) and there are techniques to limit or stop this from happening. If you smoke, strongly consider quitting. Help is available.
A little bit. Studies have found that smokers on average tend to be a slightly thinner than non-smokers, but there is no difference in height. That means there are underweight smokers, regular-weight smokers, and obese smokers; but if we average out all the smokers' weights, the average weight will be slightly lower than the average weight of non-smokers.