You're actually 100% statistically average there. e.g."On average, people who quit smoking gain about 10 pounds, according to Trina Ita, Quitline counseling supervisor for the American Cancer Society."
When you quit smoking, you can taste better, so you often eat more. Depending on how much you like cookies, that could be a lot more. You're also transitioning from one harmful addiction to a less harmful one, and while you get the nicotine there are things you aren't getting for awhile, and it can make you crave other things (weirdly, the science says we seek out carbohydrates). And while nicotine is an appetite suppressant and boosts metabolism, you often end up getting much less nicotine when vaping than you did when you were smoking. There are other things too, but you get the drift.
This generally evens out after awhile as your body finds its equilibrium. On the bright side, the stat I have in my notes is that you'd need to put on 100lbs to begin diminishing the effects on your health that switching to vaping gives you.