I'm a bit late posting this, but I thought I'd comment a little on the Horizon programme, "The Truth About Fat". It followed a (slim) doctor while she looked at some of the newer research into obesity. It's a little while since I've watched it, which is probably a good time to post because the bits that I remember are probably the bits that made the most impact.
In fat people and thin people, the levels of hormones which control hunger and satiety follow different patterns.
I think they were saying that instead of feeling hungry before meals, then full afterwards, us fatties go through the day with a sense of wanting something to eat which is never quite relieved by eating. That provides quite a neat explanation for why fatties seem to overeat and thinnies don't.
Genetics is a factor.
A big factor. Part of the programme involved a search for twins who were different weights, and what was striking was how rare they seemed to be. It was taken as read that genetics were a big influence.
Epigenetics/prenatal factors are also important.
The programme looked at the influence of your mother's diet during pregnancy: a malnourished mother is more likely to produce a child with a tendency to obesity, apparently.
Weight loss surgery has other effects beyond reducing stomach capacity
In particular, it seemed to reduce appetite dramatically.
What was perhaps most surprising was the surgeon presenting the programme had believed until then that weight was all about willpower - that slim people are slim because they have the strength of character to resist becoming fat. I suppose that can seem to be the case to somebody who has always been "naturally" slim (even though it's obviously not the case for somebody like me who has gained weight whilst keeping the same personality that I had when I was slim :)). I'm just surprised that a doctor would think that was the case.