I take great issue with you saying 'they claim to starve themselves'. The entire point is that people who are obese can feel that they are starving while eating twice the calories that they should. Some other people feel full after eating half the calories that they should. This is the almost the entire problem, the unexplained difference.
And, despite your claim, there are also demonstrable differences in the base metabolic rate between people. One of the most basic is the difference between men and women, which is usually estimated to be about 25% — men are generally accepted to have to eat about 2500 Cal per day to keep their weight, while women should only eat about 2000 Cal (for example, by the NHS [0]). There is no reason to think that, if there can be a 25% difference in the BMR between men and women, there can't be similar differences between individuals as well. Of course, you won't find anyone who eats 10k Cal and is not gaining weight (outside professional athletes, perhaps), or anyone who is eating 100 Cal and not losing weight. But it's quite plausible for someone to eat two extra pizza slices and still lose weight compared to someone else.
[0] https://www.nhs.uk/common-health-questions/food-and-diet/wha...