L8,
Force is constant from the master cylinder, since you haven't changed it.
You can calculate the relative force by measuring the difference in the cross-sectional area of the pistons.
If you distribute the constant force of the master cylinder across a larger piston you can lower the clamping force of the caliper, for the same pedal effort.
The same holds true for going to larger pads, the same force across a larger area=less PSI.
Many people updated the brakes on 911s to larger AL calipers and the pedal force and stroke became unacceptable, in addition the Al caliper flexes more that the stock steel one.
The steel caliper also has a larger heat capacity than the Al caliper, so the result was that the Al caliper faded quicker, but would recover quicker as well. Not much good when you have already slid off the track. Result, many have gone to the stock steel caliper.
They do look cool however.