I used to say that giving up smoking was really very easy, given that I'd done it so many times myself ...
Congratulations on your commitment, though, and I hope it works for you.
I (finally?) gave up in May 2013 and initially I did put on a few pounds. But in September I was diagnosed T2 so I lowered my carbs and jettisoned added sugar and sugary foods. That lost me over 1 stone in weight. Since then, I've adopted the Newcastle diet which, together with MUCH more exercise, on a planned basis, has accounted for the rest of the loss you see in my signature, below.
But to answer your question, my understanding has always been that smoking:
1) Reduces appetite, so you tend to eat less;
2) Reduces the efficiency of the alimentary tract in absorbing nutrients from the food you do eat;
3) Increases the speed of transit of the gut contents, so there's less time to absorb the nutrients in your food. I always took the often startling effect of an early morning cigarette on the bowels, as rather drastic evidence of this.
As I said earlier, good luck. The first 6 weeks are tough but it gets easier once you're past that, as you'll find that the habit tends to be broken. You also start to forget for long periods of time that you actually used to smoke, and you begin to act and feel like a non-smoker.
Eventually you get like I am now, which someone who never gives smoking a thought and gets very few very fleeting cravings. If I had to quantify them, I'd say it's now down to something like once a month, for 2 minutes each time.