It actually has to do with traction control. More and more RWD cars are using larger rotors in the rear than in the front, because the rear brakes are playing a larger role in power management, both in terms of traction control and automatic electronic "limited slip" differentials (which basically mimic an LSD by applying the brake on the wheel that has lost traction).
Bigger rotors allow more heat dissipation. At the same time, TC and ABD don't require the same clamping force as braking to a full stop, which is why you don't need a giant caliper.