The rears tend to grab first due to the weight transferring to the front wheels in a stop; the fronts get more grip and can take more brake torque without locking up. The rears 'lose' weight momentarily during a stop and thus lock up more easily, so you have to reduce the line pressure to the rears OR make them somehow produce less braking torque. The smaller caliper bores do reduce the rear braking force, all else being equal. And so does making the coefficient of friction (CoF) of the rears pads lower than the front pads CoF.
