She was a model before her fame exploded so I've no doubt she's naturally really slim, you can see she has those genes, and then she has the kind of body that responds really well to exercise and diet. The exercise makes her even slimmer and more lean than she already is naturally.
Fair play to her, she does look amazing even if her figure is not achievable for 90% of women. I'd eat similarly to her, haphardly, not great but also not much and always rushing around. It's knocked a lot of weight off me in the last few years and made me realise how people who are naturally slim often just tend to be like this, not a huge appetite and not a "foodie" i guess. If I have a big restaurant meal I'll naturally not be that hungry for a few days after. I just don't have the inclination to exercise the way she does!