I have seen how brake rotors have become smaller after servicing many GM and Ford cars since 1975. My 1995 Buick Regal had 11-inch rotors, yet never gave me problems in 21 years of service. As a fact, the pads lasted long, as they were changed only twice in those 21 years (the dealer did it once and I did the second one at home). My present 2009 Chevy Equinox also has small brake rotors yet it also stops perfectly after eight years (still has original pads!).
I expect the brake systems to get smaller (and weigh less) with hybrids and electrics because the motor regeneration will do most of the "speed reduction" (action) until the last few feet (or meters) where the hydraulic brakes come in. That is only low speed braking and you need less effort and materials to do that. As a side benefit, the wheel weight is less, the cost of pad replacements will drop, and the rotors will last the life of the vehicle. The only exception will be if the driver does many "panic stops" due to terrible driving, not good for any vehicle either gas or electric!
Extra: There is a prototype high speed car, made from the fuselage of the F-104 supersonic jet fighter, that will try to break the British car's record as the world's fastest car. The brakes are actually inductive, using a pair of aluminum rotors (one fixed on the body and the other on the axle) that generate a huge eddy current, and an induced and opposing magnetic field at high speed when the rotors are pressed together, producing the reverse torque needed to stop the car so that it has no hydraulic brakes and doesn't need a parachute, either. It is called the "Eagle".