The bore size ( diameter ) has no effect on whether a brake pedal feels soft or not.
A soft or spongy pedal comes from brake hoses "swelling" under the pressure and/ or air in the system. Pumping the brakes to bleed the air out will actually cause the air to stay entrained in the fluid. So try to limit pumping the pedal as much as possible. Also, some years back there were bulkhead fittings that because of their shape would hold air at the top of the fitting. These were used to pass through the frame typically, there was a "raised" section in the middle that would actually act as a reservoir and hold air.

The larger diameter brake piston will give you more surface area - so more "brake horsepower". Always a good thing!