OK, first of all, no manufacturer makes a car with equal size discs all the way around the car. The front brakes are always larger because they take the most of the inertia of forward momentum when stopping. It has nothing to do with whether it's front, rear, or 4 wheel drive. Also, most manufacturers don't make vented rear discs for brakes, as again, the front brakes are taking most of the inertia and also get hotter, so thus they are vented to cool more quickly. I'm just glad the Koup has 4 wheel Antilock Disc's, instead of Front Disc/Rear Drum Brakes. From all of the road tests I've read, and seen videos of, Everyone seems to be impressed with the Koup's stopping distance (Like 120' 60-0), Fade, and braking feel, so I'd leave mine alone, but that's just me. Besides I don't want it to affect my warranty.