It's not just fat people, it's a popular trend in makeup in general. If you google "contouring" you will see a lot of pictures of thin girls.  I lightly contour my cheeks sometimes because despite having a healthy BMI of 21 I just have a round face and I feel like light contouring makes it look less round :P If you have a wide nose contouring can also be a good technique to add some definition. Some people definitely overdo it but when done well it can enhance features without looking fake.