I gained 3 stone over a long time simply eating a very small surplus (50 calories a day) over a long time. People are -generally- good at eating the right amount of maintain their weight, but even a tiny margin over a long time can add up. Overweight people I know don't actually eat that much more than your average person!

Oh, and I lost that weight and more simply by - duh - eating less than my body needed. It's not an exact science, but it does work. If you don't think it does, you either have a very rare medical condition, or (sorry for the tough news) just need to eat less and exercise more, like everybody else.