How are we able to selectively inflate our cheeks?
Muscles can only "pull" never "push" - that's just how they are organized at a molecular level. We can only push things because we use joints as levers: if you "pull" on the outside surface of your arms, you can push things out with your hands because that pull acts to extend the forearms.

It's not really possible to puff out your cheeks with muscle, because there is nothing floating in the space outside your face to pull against. Your cheeks puff up a bit when you smile, but I think you will agree that's a completely different type of "puffing" and comes from gathering up a bunch of tissue.

However, it's fairly simple to contract muscles in your face (specifically the buccinator) to prevent a cheek from bulging out by tightening the muscles in the cheek. The air pressure you are producing is pretty minimal, so it doesn't take much strength. The cheek bulges out when the muscles are relaxed, and doesn't bulge when the muscles are contracted.