^ Look at the rear brake discs on the RS3.
(But I can answer the question from a technical perspective. Although aesthetically, it would be nice to have bigger/the same discs on the rear as front, on the RS3 it's just not necessary, while the BMW weight distribution is closer to 50:50, the RS3 isn't, so very little will be demanded from the rear brakes, especially under heavy braking. The back end goes quite light - so smaller brakes are all that are necessary. Keeps costs down too. Also, with Haldex, you're still braking a front wheel drive car - so the back brakes play no part in retardation of the driveline.)