The part many people miss in their brake calculations is the one part of the system that`s not bolted to the car...your leg! Yes a larger dia. piston has more sweep area can make more pressure with the same travel all else being EQUAL. But it also displaces more fluid than a smaller piston and achieves the same amount of clamping force at the caliper with less travel. Less travel+more volume per increment of travel equals higher effort required to push the piston that distance. Think of it as hydraulic leverage. In other words a larger piston trades pedal effort for shorter travel. End result if you go too large is that while the master cyl. is capable of higher pressure your leg is incapable of pushing it hard enough to achieve it. So you may have a rock hard pedal but won`t even be able to lock the wheels up. The ideal dia. master may provide less peak pressure and you`ll have longer pedal travel but you`ll be able to make full use of it and the car will actually stop much better.
Now with a vacume booster you can run a larger dia. master without the liability of a super heavy pedal. A good hydraboost goes even further. Think of it as a bionic leg.
Now you can run a much larger dia. master ,get even higher pressure/clamping force at the calipers and still have very reasonable (even pleasant) pedal effort.
Example we`re just finishing up a `61 2dr. Suburban ProTouring project "Project ProTow". This thing is HUGE and heavy. We put big Baer brakes all around and Hydratech hydraboost to stop this monster. It`s amazing,it stops like a sports car. Nicely modulated pedal feel,light effort and panic stops make you dizzy (literally!). It`s damn hard to get all 3 with manual brakes in something that BIG. Mark SC&C