I've been fat my whole life.
I'm very short, and just don't seem to have whatever it is that tells you when you've eaten enough (I had it when pregnant, when suddenly it was easy, and I didn't put on any weight).
If I eat like a normal adult, I put on weight. I have to eat like my children to maintain, because I basically am the size of a child (or rather should be - I'm obese). It's very hard, because I don't get the signal, I have to manually monitor what I'm eating and stop when I think I've eaten enough, and as anyone who's dieted knows, it's very easy to over-estimate.
The only times I've lost weight were when I had Glandular fever and could't physically eat, with low carbing (which takes so much effort to plan) and with fasting (lost 10kg on fast 800 - but again, requires planning effort).
I've been a regular gym-goer/exerciser - 2 hours of walking/jogging/day in lockdown - and that doesn't knock the weight off (although I do feel better for it) - I lift heavy weights with little difficulty, have excellent low-level stamina, but only eating strictly controlled amounts works.
So in summary - is it a choice? Kinda. I think it's like a line I just heard on a TV program (this was around privilege, but it works here too) - it's not that you make good choices, it's that you have good choices. ie. most slim people are like that with a lower level of effort. It would be like me telling someone they were making bad choices because they were struggling to lift a heavy suitcase - I find it easy, other people find it much harder.