Good question! Rotor DIAMETER is often an accomodation to the wheel assembly the rotor is being mounted to. Go out and take a good look at your front and rear rotors. You'll notice that the swept area (the area contacted by the pads) and the thickness of the front rotor is greater than the rear. You'll always find "beefier" brake components up front. That's where all the weight of a vehicle is concentrated in a stop, irregardless of front or rear drive. Your rear brakes are also somewhat adjustable courtesy of a proportioning valve that reacts usually to the angle of the rear axle to the pavement, dynamically adjusting the pressure available to the rear brake.

So the wheels getting the power don't necessarily get the brakes. Just look at a motorcycle, double discs up front and a single disc or drum at the rear. You almost exclusively use the front brake.