this is what i know to be true about brakes, simplified:

hydraulic pressure in a system is constant, no matter how many pistons there are. more piston area = more force, be it via one large piston or many small ones. it doesnt matter.

brakes are energy conversion 'machines'. they take the kinetic engergy of a car and convert it to heat engergy. how much heat? 1/2mv^2's worth.

to do the energy conversion brakes do work. F*d = work. d is the circumfrence of the rotor*# of rotor revolutions. F is the friction force of the brake pads. F = Fp*u. u is the coeff of friction of the pads on steel, and Fp is the piston force.

# of rotor revolutions = # tire revolutions. This is why stopping distance reduction is related to rotor size. The more linear distance the pads act over per revolution (larger rotor), the more work is done converting energy per revolution.

Likewise shorter stopping distances can be achived by changing the F in the F*d equation. F itself is made of Fp and u. So grippier pads (more u) or more piston force Fp(via more piston area) are the other two ways of making brakes convert energy better.

The 'lever arm' people like to talk about is a red herring. All that does is change the braking 'feel' or onset of force to the rotor. As long as the brakes are not locking or the tires skidding, the lever arm has no impact on braking.

to bring it all back around full circle, 1/2mv^2=F*d/s (s is the time it takes to stop)

Finally to answer the original poster's question again...rotor circumfrence is Pi * D . D is the rotor diameter. when comparing the ratio of two rotor sizes, the Pi's cancel, leaving only the diameters left to matter.

i hope this clears up the confusion on how brakes function. provided the brakes aren't fading, tires skidding, or the brakes locked...the above applies.