OK let me try to explain as best as I can the differences between 2 piston vs 4 piston systems and how pressure and volume come into play. Let's start with the total piston area. Which has more total area, one 1/2 inch circle or two 1/4 inch circles?? HINT: two 1/4 circles fit inside one 1/2 inch circle. So hopefully you came to the conclusion that 4 piston calipers have less area than 2 piston calipers. Now theoretically either caliper has to travel the same distance to make contact with the rotor via the brake pads, and VOLUME is area times distance, so the 4 piston calipers have slightly less total volume (not 1/2 like in my example) than the 2 piston. Therefore, for the same force exerted on the brake lever to give the same force on the brakes a smaller area master cylinder is needed.
As already stated, smaller area master cylinders give higher psi. Just changing the master gives you a longer stroke but higher psi when you squeeze the handle with the same force. So, just changing the master to a smaller diameter gives the stock brakes more of a grabby response but with a much longer lever movement. Just changing the calipers without changing the master gives a harder feeling lever with a shorter stroke but less pressure i.e. you have to squeeze harder to get the same braking power.
Hope this sheds some light on the subject...