Maybe it's about portion size.
But yes, what you want is crucial. So thin people who say they eat what they want may not actually want much.
I'm not sure what you mean by 'want'. Too subjective.
I have a hearty appetite, but I don't eat what I could (before I'd feel stuffed.) So as an example, if I make a pudding I'll have a portion that fits in a ramekin dish, rather than a soup bowl/plate.
If I'm out for lunch, I rarely have dessert even though I'd love it.
At home if we have a roast dinner, I'll have roast potatoes, but one or two small ones, whereas DH might have 6.
I've a friend who when she eats soup for lunch, always has 2 slices of bread with it or rolls. (She's unhappy with her weight, by the way.)
I don't and will fill up on fruit or yoghurt. Small differences that make a difference long term.
For me, the trade off is eating smaller portions so I don't end up having to buy bigger clothes (all my weight goes right to my waist and belly) but not totally depriving myself.