Larger Rotor Brakes

Some good answers above, but the real increase in braking efficiency is due to the greater 'swept area' of the larger rotor. Because of the increase in diameter, the pad is in contact with more area (square inches of contact) of the rotor per revolution of the wheel. The larger the diameter of the rotor, the more area the pad will contact during a single revolution of the rotor. Therefore, more 'swept area'.