Not that it's better at all, but some guys are concerned with how a larger wheel with a smaller brake appears. I've heard of guys running 18" wheels up front and 20" wheels in the rear, and wanting their brakes to appear proportional. Not that it's better in any way, and not that I would do it personally, but I have heard of stranger things....
The rotors at the rear are smaller because you don't need as much brake at the back.. my race car has 15" rotors with a Brembo Endurance Caliper up front... at the rear I use 14" rotor with the same calipers a Ferrari F 50 uses up front... during a race the bias will get adjusted to compensate for changes in braking due primarily to tire conditions...
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Burn Rubber Not Your Soul
A bunch of Camaro's
Porsche 962
Porsche 935
Porsche IROC series RSR (Fittipaldi)
As Thunder already referenced, the amount of rear force required to slow the vehicle is drastically reduced. Besides the obvious front to rear weight bias, as soon as you tap the brake pedal you move 20-30% of the rear weight forward increasing the amount of front brake required. If you had equal braking F to R, you would have a very tail-happy vehicle.