having big brakes on the front is one thing, but the better they are on the front then the less hard you will be pressing them, so the rears will be getting less power to them, altho a bias valve can help sort this a little bigger and better rear brakes help as they are more controllable, small drums are very on off and are a bitch to keep perfectly set up,

a mate of mine has a 205 rally car with massive discs on the front and standard 1.9 gti discs on the rear, he then got a kit that made the rears 300 mm using the standard calipers and the difference was awsome apparently, in his words, the brakes just all of a sudden worked,

thats why ill be going big discs on the rear eventually