HC...Dave is right. While there may be some people who have a "glandular" problem, most people are fat for the reasons he gave. Look at the huge profusion on fast food outlets and takeaways that we see today.
I live in a small village, with a population of about 5000. But we have 2 chip shops, 2 chinese takeaways, 3 indian takeaways, 2 "kebab" places, and now a brand new Domino's Pizza. Lots of working class places that I visit for my work have a similar quota, if not more. I am not being hypocritical here, as I have a chinese takeaway on the first Friday of the month. But with all these places selling high fat and high calorie food, its quite clear what is causing this modern epidemic of obesity. And its not genetic !