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).
Possibly some of them could also be unintentionally countering the impacts through things like self-medicating with caffeine.
I have no personal experience with GLP-1s but suddenly quitting caffeine after decades of daily use was pretty eye-opening in terms of how much it was boosting my baseline mood. And I'm not talking about being cranky for a couple of days after quitting, I'm talking about a very noticeable sustained change from when I was a frequent caffeine user to when I stopped having it altogether.