But all that is just justifying obesity.
No. I am presenting a purely scientific explanation of causation. You are concerned with blaming fatties, which is a moral and unscientific position.
It makes an unspoken assumption that everyone needs to (or should be able to) eat the same amount - people don't need to (or not should be able to) because people's metabolism and activity are different. People choose to eat more than their body requires.
I am making no assumptions about people "should" eat, because that is an unscientific, moral question. Science has nothing to do with "shoulds". You and all those critical of the OP are making the purely moral arguments based in assumed "shoulds." You presume that people "should" only consume the amount their body requires for fuel, so you judge people negatively who violate your preference and then judge those who don't share your judgment of fatties. This is what makes you and others unable to separate a causal explanation from a "justification". It is the confusion between these that people seeking to place moral blame on others always get upset about when someone provides a scientific explanation for something that includes factors that are not under volitional control and don't lend themselves to moral judgment. It is the same as the religionist who gets upset at science for "justifying" criminality by explaining the causal influence of childhood abuse on future criminal behavior.
Some marathon runner in training can consume over 5000 calories/day and lose weight. This doesn't mean that everyone should be able to eat 5000 calories/day and lose weight, not even the marathon runner when he is not in training.
Eating more than the body requires is a choice, not a requirement. And what one person eats has no effect on the weight of another person unless that first person is eating that other person's food. So comparing what different people eat tells us nothing about why some individual is overweight. That individual would be overweight because they choose to consume more calories than they burn to sustain whatever their normal activity is - not how much less they eat than that marathon runner or anyone else.
Almost no one, including "normal" weight people choose what they eat merely based upon matching it to what their body requires. So that is not a difference between normal and overweight people. Consumption for most people is massively determined by past socialization, current social practices of serving sizes and meal frequency, food availability, sensory pleasure, etc.. Whether those factors lead to consumption that matches a person's bodily fuel requirements is often a matter of random luck. The less overweight person is often not thinking any more about, trying any harder to match it up, exerting any more self control, etc..
In addition, all of these factors that drive motives to consume are highly variable between people and not under direct control. For example, many people are born with taste/smell systems or brain reward systems that mean they just don't enjoy food as much as others. They don't eat less because they have more control in trying to match their needs. They eat less because they don't like most foods (often for innate biological reasons) and if they also have a slow metabolism its just luck that this leads to a better matching of their fuel needs. IOW, the fatter person is often exerting far more self-control and more effort to try and match their intake to their bodies needs, but those are not enough to compensate for all the factors outside their control that make this task far far more difficult for them to achieve than their thinner friend who by random dumb luck find themselves in a body whose caloric needs better match the environmental factors that shape caloric intake.