It’s an enormous divide between those two categories, and we have about a thousand behavioral contributors — mant of them social and difficult to escape —- that plainly play a bigger role than biological variation.
If you can’t manage to lose those last 20 pounds, it might be some underlying metabolism or gut difference. If it’s 59 or 100 pounds, then there’s almost certainly something else at play and an honest diet and activity survey are very likely to raise concerns.
And the variation can amount to quite a bit. All it takes is time. An excess 100 calories a day adds up to 10 excess lbs in a year. 5 years down the road and you are 50 lbs overweight.
Moreover : so how ex-obeses who became more "average" do ? A genes therapy? And I am not referring to people who had some surgery but those who could lose tens of kgs.
From another angle : so obese people weighting 200 kg also eat like anyone else, but just have a very inefficient metabolism? NO.
And yes, you can become overweight eating the same thing as someone else simply by having a more efficient metabolism. Unless you believe that there is absolutely no variation between humans, which would be ridiculous. All it takes is a 1% difference in BMR to amount to 25 excess pounds over a year's time all else being equal.