Other than bigger pistons in the front calipers your other option is smaller pistons in the rear.

Are they rear drums or rear discs? My old Astra had worn rear discs which had a tendency to grab and lock. All fixed with new rotors and correct adjustment of the handbrake. Drums can grab if the hinge points aren't right, I've never had a problem like that with them though so I'd just check it's all assembled correctly.

Dunno if that helps. Good luck!