It depends on what you mean by "in your control".
Obviously (1) you choose what to eat and (2) if you eat little enough then you will get thin, and if you eat enough then you will get fat.
(But also: (a') you choose what to do and (2') if you never do anything that violates your values then you will be some sort of saint. But most people are not any sort of saint: we sometimes do hurtful things even though we think we shouldn't, or stay up later than is good for us, etc.)
But our bodies and brains are evolution-designed to make very sure we don't accidentally starve to death. So we feel hunger, and hunger is very difficult for us to ignore. And if something is miscalibrated so that the level of calorie intake at which you stop feeling hungry is enough to make you fat, then you will either get fat or feel constantly hungry.
And even if nothing is miscalibrated in that particular way, human brains are annoyingly good at acquiring habits that are difficult to break even when they are bad for us.
All of which means that while anyone can in principle keep themselves neither too thin nor too fat, according to whatever standards they find appropriate for that, doing so is much much more difficult for some people than for others, and if a thing is difficult enough to do and requires doing all the time I think it is no longer reasonable to say that doing or not doing it is "entirely in your control".