There has to be a reason for that right?!
The sales guy (Charles) talked to the engineers and they said to keep the braking bias of the OEM braking system. Because the rear rotor is larger than the front rotor in the OEM setup (IT IS NOT, the front is slightly larger than the rear), they decided to keep the front rotor smaller.
Now that being said, the rotor size isn't everything when it comes to stopping -- yes more friction surface, but the front is a 6 piston and the rear is a 4 piston.
But wait -- the front 6 piston is almost identical (if not smaller) than the rear 4 piston caliper size! The rear pistons are larger than the front! Confused yet? I am.
Brembo, (which I believe owns AP Racing), did their own research (as did StopTech), and found a rear brake kit isn't necessary and leaving it at stock 12 or 13 inches (whatever it is) and stock caliper is fine, and putting a large 4 piston, 15" rotor setup in the front works good with the brake bias of the FX since as in most cars, the braking is in the front.
R&D is expensive for a rear kit due to the eBrake (which is why the AP Racing rear kit cost over $3,000), so both Brembo and StopTech did not create one due to lack of demand.
