Exercise is probably the key, as people here have said. I have a fat cat, and restricting her to small amounts of dry diet food has done nothing to help her lose weight - and it just made her meaner than she already was. I suspect that just like humans, some cats are just prone to being overweight.
I have noticed with my fat cat that no matter how much food she has access to (very limited, or always accessible) she stays the same weight. I prefer her happy and fat rather than mean and fat, so I no longer restrict her food.