Are they compressed or cut off? If compressed, what values are you using for DEFAULT_STEPS_PER_UNIT in configuration.h? Another thing to check would be to make sure the tubing shaft couplers on the Z motors aren't slipping.
FWIW, both of these posts suggest the heat bed on the 12-inch printer draws a LOT of current. More than just the linear increase expected due to the increase in heater area.