You can get too complicated with this;
Every rev takes in the same volume of air because the piston goes up and down the same amount on each rev.
As the air pressure decreases the mass of air (and thus the number of oxygen molecules) in a specific volume decreases.
With less oxygen taken into the pot at each stroke it is obvious that each stroke will produce less power.
Perfect, shimples. And the prop does the same, same volume but less molecules, less molecules = less pull/push.