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I was reading up on forces on hydraulic/pneumatic cylinders, and I've been thinking of this for a while:

So for a pneumatic cylinder, the force on the piston is simply:
P = F/a
F = P x a
where a = area of the piston that the air pressure is acting on.

So what would happen if the piston is chamfered, thus increasing the surface area?

See above, (red circles are o-rings). In a closed system, my guess is :

1. The overall volume has increased, decreasing the pressure inside
2. However, surface area is increased due to the chamfer
3. Therefore the resulting downward force is the same.

Is this idea correct, or have I missed something?
Edit : The next step is, given the same air pressure, how could we estimate the increase in downward force?