I'm slim but I eat regular amounts I think, big bowl of cereal for breakfast; toast, fruit and yoghurt/crisps for lunch; then a cooked dinner. Usually with biscuits and more fruit in between. And sweets/chocolate like every night. But I'm a reaaaally slow eater so I don't know if that has anything to do with it... apparently if you rush your food then you get prone to putting on weight? (don't know if this has any truth in it at all...), but my friend eats so quickly and he's a lot bigger than me, so there could be something in it?
And when I'm at uni I totally walk everywhere and go to the gym once or twice a week