I also used to smoke. In fact, step 1 on the path to losing weight was quitting smoking. It’s next to impossible to have the motivation to be active when you smoke.
The stomach reducing surgery is highly effective, an egg sized meal makes you full - although I think the opposite can happen - too small a stomach in developing kids. I have family members where their kids eat two bites and they’re finished - they’re also way small for their age. I don’t think they’ve ever had a full meal. So I think the stomach size is key to being healthy.
Quitting smoking for me was the path to gaining weight. I found myself 20 pounds heavier after a month or two. I basically switch from smoking to over eating. I managed to lose it again, though.
That does not appear to always be true-most of the guys I work with smoke to some extent-none of us smoke at work, though-it is a customer service thing, like not dropping F bombs in front of a customer-I smoke 2-4 cigarettes most evenings after dinner-I work on construction jobsites, where there is no job that does not require considerable physical activity-including mine. All individuals are different as far as energy levels-like the kids you describe with the tiny stomachs...