It seems to me that the calories that I eat less the calories that I burn off determine what I weigh. It seems to be a fairly precise relationship. It is probably about the same for most people ie if they eat the same as I eat then they will weigh about the same as I do. There are a few medical conditions that may substantially change the relationship but it appears that they are few and far between, perhaps 1 in 10,000.
I think the biggest varations come in the other part of the chain, that is the motivation to eat phase. There are perhaps two parts to this:
I think that it is in these two parts that there is the most variation in people for example some people will and do what they think is right based on logical reasoning, whereas others will do what their emotions suggest they should do. Just this one variation could be the difference that means we eat an extra 1000 calories a day and perhaps weigh 20 pounds less.
Another wildy guessed at idea would be that some people subconsciously process what it is reading what is going on in the body and decide that they shouldn't eat anything and send that message on, whilst another person reads exactly the same body signals and determines that they should eat a lot of something sweet straightaway. I have no way of knowing what is going on for someone else as they have no way of knowing what is going on for me. But in ths example this could easily lead to a person eating 1000 calories more than other.
For some it is a lot tougher than others and I am pretty sure that for me the variables are mostly in the mind.