Quote Originally Posted by Born Again View Post
Not quite. Splitting hairs, inertia is the tendency of an object to remain in the same state, either stationary or in uniform motion, resisting change.
To really split hairs, inertia is a property of a moving mass only if that movement is uniform, which the piston is not - it's either accelerating or decelerating (apart from an instant before it bounces).

The important point is that it is the reduced momentum of the lighter weight piston that makes it less able to overcome opposition from the compressed air.