My entire life I have been underweight and also ridiculously hooked on caffeine. At work I would easily go through 5-8 cups of coffee in a day.
I quit caffeine to become a parent a few years ago and suddenly I am able to put on fat and muscle so much easier.
I think the fact that 95% of our society consumes one of the best weight loss drugs (caffeine) anyway really skews a substantial portion of the data.
Looks like it's typical to gain 5-10lbs when quitting smoking.
Edit: pulled up the data and there's been a 30lb increase in median weight in America, so (loosely guessing) the smoking effect is probably somewhere between 5 and 10% of that change.
Nicotine is a very useful drug that is sadly so very, very bad for you.
It has weight loss impacts, acts as a stimulant and is actually one of the better non-prescribed treatments for ADHD (as well as being super associated with schizophrenia).
After I got my ADHD diagnoses I noted that this was always called out in the literature, and I have been addicted to nicotine for the past 25 years or so (on a vape now, which appears to significantly reduce some of the bad side effects, but is still probably not good for me).