Matt, nearly right.
A FL is mostly front wheel drive, with a small %age going to the back at all times, courtesy of the "slip" in the VCU. When the front whhels loose traction the VCU sense this and locks up, sending more of the drive to the rear.
If you stick the largest tyre on the front then the differential ratio between the 2 axles is such that it can overlaod the VCU, potentially "cooking" it and then damaging the IRD and/or rear diff.
Hope that helps.