One of the many reasons smokers give for not quitting is that smoking controls their weight, and that quitting will cause you to gain a bunch of weight back.
It's not only possible, but it's quite easy to quit smoking and not gain weight.
The idea that smoking makes you lose weight is a false one.
The dip in nicotine levels in your blood that causes you to want to smoke also feels like an empty stomach, so many people end up overeating as smokers.
Food also doesn't taste as good (smoking damages your taste buds) so you tend to eat more fatty and greasy foods.
Still, the way that you quit affects if you will end up overeating as a non-smoker.
When quitting, some people replace cigarettes with hard candy and the like.
This isn't as effective at helping you quit, and that's where you hear a lot of people complaining about jumping ten waist sizes when they quit: it's not because you quit, but because you're eating two bags of candy every day! When you quit smoking properly, you'll not only not gain weight, but find that you're able to keep the exact same lifestyle you currently have, except that you have more energy and food tastes a lot better (that's not an excuse to eat more of it!) Despite what you may think, smoking isn't a lifestyle and there aren't any upsides to it.
It's just a detraction that can be applied (and removed) from any walk of life.