I've been slim all my life, I'm 48 now and a size 10, I think I just have a really healthy relationship with food. I love food and cooking, have an extremely varied diet, love planning menus and cooking for friends, eating out, all of it!
But, I only eat when I'm hungry and stop when I'm full, that's the only secret really, I've got classic 'eyes bigger than belly' syndrome, I only naturally eat fairly small portions, it's not a conscious choice, I just stop eating when I've had enough.
We opened some chocolates last night and I had two which was very enjoyable, I didn't think 'I've got to stop eating chocolate now or I'll get fat', just, 'That was nice. I've had enough now'.
A PP mentioned gut-flora, I do think I've got a nice healthy gut, never have any problems with it, I presume it's because I eat a massively wide range of foods. So that may be a contributory factor.