Disc brakes don't actually require more fluid to operate. If you look at drum brakes, you will always see an adjuster on the brake shoes. Disc calipers have no adjustment for wear of the pads. As the pads wear down, there is more and more fluid sitting in the caliper when the brakes are not applied. The reservoir is larger so that there is no danger of the fluid running low during the life of the pads. If you keep a disc reservoir completely full, you will have to remove fluid when you change the pads. That doesn't happen with drums when you change the shoes.
Piston size is not necessarily bigger on disc brake MC. Size is more of a factor of power booster or non-power. If you look at most older drum/drum brakes they are non-power. With some exceptions, disc/drum and disc/disc have a power booster. Larger MC pistons require more
input pressure for the same line pressure as a smaller piston. The booster allows less pressure from your foot to overcome the need of the larger piston.
Look at a disc/drum MC. The bore diameter is the same for the disc portion and the drum.
The web site below has as much info as any.
http://www.mpbrakes.com/technical-support/configurations.cfmThe info you need has more to do with pressure than volume.
Non-power MC's have a deep hole for the brake rod, so the rod doesn't fall out when you take your foot off the pedal. Most power MC's have a shallow hole because the rod in front of the booster is short. I say most, as there are some that use a longer rod.
