Why do Some People Have Apple Cheeks and Others Fat, Flat Cheeks?

You bring up a good question. I find that a big factor in patient not having "full" cheeks is that their cheek bones under-project the overlying soft tissues. Even if there's a normal amount of fullness of this soft tissue the cheeks will appear flat.

Granted there is also quite a bit of variation as to how full the soft tissue is as well. Some people have more underlying fat in their face than others.

Another effect of having flatter bone structure is that the overlying skin tends to be more affected by gravity (as evidenced by sagging).

Youthful faces are faces with some fullness. But that fullness is in the appropriate areas. as we age, we lose fat, but the fat can also shift. because fat grafting has recently become so popular, many physicians without much expertise are now performing fat grafting. Often the results are less than ideal. The goal of fat grafting should not just be to add fullness to the face, but to add fullness in a way that makes the face look youthful and beautiful. The change should be subtle and natural looking. Observers ideally will see a fresher, more youthful look that does not scream, "I've had fat grafting" 

The shape of the cheek bones, the Maxilla and Zygomatic Arch are the most important determinant of a Prominent Cheek Bone appearance.

The Apple Cheek refers to the nice plump youthful appearance of the cheek so often seen in children and referred to as "chubby cheeks.' This fullness is caused by a pad of fat called the Malar Fat Pad.

A full prominent curved cheek bone is most important for the full cheek. People with flat cheeks have an underdeveloped cheek bone a condition called Maxillary Hypoplasia-more common in Asia and Africa.

As we age, the Facial Fat Atrophies-is lost-and the apple cheek is lost.

Fat Transfer can plump the cheek and restore the apple cheek. In cases of severe Maxillary Hypoplasia Cheek Implants may be required