@ Cornellian

Of course it's down to metabolism. But metabolism isn't set, it can be altered based on the muscle mass and eating habits of the individual (I'm leaving out various disorders and stuff; I'm happy to admit I have zero knowledge in the field of genetic disorders). That is why, for example, body-builders can stay lean whilst eating 6000+ calories per day - with the amount of muscle they have, their bodies are like metabolic furnaces.

Also, it's generally recommended to eat 6 meals / day, not 3 small ones, in order to maintain a constant metabolism. Usually, these consist of your 3 main meals with 3 smaller meals / snack meals between.

There is a hereditary element, yes. Some people don't break down calories/fat/glucose/chocolate digestives/etc. at the same rate as other people. So, in order to NOT put on weight, they have to regulate their intake. Soooooo, if calories intake is greater than the expenditure (BMR * standard work load factor + exercise allowance - some daft constant I can't remember), based on a standard person with no genetic disorders, mutations, etc. then the calories will get stored. As blubber.

And thank-you very much, I have done quite a bit of research on this topic.