it's the size of the piston. because the piston moves the fluid this has a direct correlation on pedal feel in conjunction with caliper piston size and amount.
also, you're unlikely to get a firmer pedal with aftermarket multi piston calipers. more pistons, more area, and it gets less firm. it's too hot out to explain the math right now.
either way, you need a disc/disc master. i'd upgrade to the modern two bolt master like from dr. diff