I eat a ton and my weight never gets over 215. How do they do it?
Here's my armchair psychologist take on it
Most people have a self image of themselves that allows them a certain degree of wiggle room in how to behave. I weigh 170lbs as an example, and my behaviour and habits will only drastically change if I creep 15lbs outside of that range either side. If I start to hit 180, then all my eating and physical fitness changes. it's quite frustrating for me actually because I don't seem to have the motivation to improve physically as long as I don't feel very bad about my self image.
However, that assumes I know how to correct the problem. A lot of people don't. They're either misinformed or ignorant. They tend to overestimate the importance of a smal session of physical activity (e.g. a 30 minute run) and underestimate the damage of a "small" victory snack. And so their corrective behaviour doesn't do enough. So they can't get back into that self image perfectly
Metabolism might be a small factor, but it's probably more about personal habits and physical activity.
With enough time, that self image changes. So at age 20 it's 170, at age 25 it's 175, and so on. Impercetible changes over a long period of time that add up to a lot more
Most people also have a reflex that tells them when they've eaten too much or not enough. Some people don't seem to have that, or theirs is broken, or they can just ignore it. Maybe they're just hungry all the time and never feeel full.
Personality disorders, such as dysmorphia, self harm, etc.