How much of an effect can metabolism actually have though? That's the crucial part of this. If someone claims to be 300lbs and not losing weight eating say 1700 calories a day (made up numbers) I would just say they are either incorrectly calculating or not being truthful. By virtue of thermodynamics if they have a certain amount of tissue then they require a certain amount of base calories to maintain that tissue.

Essentially, I think people misjudge how much of an effect metabolism plays compared to other factors, misreporting or miscalculating caloric intake, self deception, etc.

Body composition is another thing entirely and involves going a layer deeper to macronutrient breakdowns and exercise variants.