The model is vegan, vegetarian, or eats a high carb and low fat diet. 
Actually,
high fat and
low carb would lend more to such a lithe figure.
Fat, all by itself, will not put weight on you. Carbohydrates turn to sugar in your body, and are stored as fat if they are in excess of what you burn off.
I regularly eat a high-protein (with the fat inherent to such things as bacon, butter, and so on) diet with as little carbs as I can.