What? It is a choice how much you eat.
You might be bedbound and not in control of your own food. But, the staff could just give you less food.
There is no medical condition that will cause someone in a locked room given a calorie restricted diet to gain weight. That is against the laws of physics.
It's fine to say "losing weight is very very very hard". But it's incorrect to say "being overweight is not a choice".