Originally Posted by TimMc
The "The rear brakes do not do little braking because they're small. They're small because they do little braking." is certainly correct. Weight transfer under braking means that there's little weight over the rears so little force needed before lockup.
And to add to that, they "can" be small because they do less braking than front brakes. Less braking means less heat, and that means a smaller rotor "can" be used as the rotor size is purely a function a heat, not braking performance. Same for caliper (not piston) and pad size. Larger caliper and pads do not equal better braking performance per se. They just generally insure great thermal consistency during braking, which means harder braking for longer.