Everyone seems to want a large-bore master cylinder, but that means your pedal effort is higher. The reason to go bigger is if you have too much pedal travel. If you move to larger caliper pistons, or swap from drum to disc, the brakes need more fluid to move, but I think the common perception is backwards. Hydraulics are similar to a gear ratio. The force on the brakes is proportional to the ratio of piston diameters, so as the master bore gets bigger, you have less pedal travel, but less braking force.