Originally Posted by bradford
The physical size of the rotors and calipers has little to do with "how much" braking is done by that particular set of brakes. Piston size determines brake bias. And I'm pretty sure that the engineers at BMW understand how to compute optimum bias.
This is not quite correct. Piston size is one variable but there are so many others including; ratio of master cylinder piston to slave cylinder pistons, number of slave pistons, the distance these pistons are acting out from the centre of the wheel/disc carrier, pad material etc, etc.
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.