by bigger im assuming that your just talking about the size of the disc, which wont have much to do with the rear locking up before the fronts
its when the rear calipers are too small (in piston area) compared to the front that they lock up not when they are too big

id say they would be fine when using 120y fronts bias wise, but wouldnt suggest keeping 120y brakes if you have the need for a r31 diff anyway