I am not too sure what volume has to do with this unless the MC is woefully wrong for the design. Disc did require larger reservoir to account for pad wear and the larger volume of fluid required to replace as piston moved outward from disc wear.

The 4 piston versus sliding is just physics. Same as pushing on a wall. Equal and opposite reaction to force, unless you are actually moving the wall, or in this case the rotor. So you calculate 4 piston based on the 2 pistons force per side. Same for 1 piston, force on that side, is equally applied on the other side.

The 69 B use 4 2" pistons so you had 2 2" pistons on one side that actually had a larger area then the 70 KH 2.75" piston. 8 pie, vice 7.5 pie. They both used the same 1 1/8" bore MC.

The whole reason braking was better is the pad friction was improved and the pads had 50% greater friction than earlier year pads. So even though applied force decreased from the smaller area, braking force increased.

The Master training documents on the web in 69 and 70 have great info on hydraulic and design info on the brake system. Of course they were power brakes required during that period for most applications.