Correct me if I'm wrong, (I'm sure somebody will), but the ABS shouldn't be affected by the size of the brake disc. The ABS sensor is located behind the hub, not on the rotor. Differential in wheel speed is what triggers the ABS via a gear-like magnetic sensor behind the hub.
Brake proportioning is a different matter, as the larger rear swept surface would cause increased friction in the rear, effectively increasing the proportion to the rear. This might be cured with a manual brake bias valve, ala' race car, but not sure how this might work with ABS.
Just some food for thought...
