I also use the FCD on some of my cartridges. Do you feel any resistance when the cartridge enters the FCD? If it is sizing down the case because it is over size then you will feel some resistance as the cartridge enters the die and before it actually crimps the case. That resistance will be the sizing ring in the die sizing down the cartridge. If you feel no resistance then it shouldn't be sizing it down. Have you measured the diameter of the case before using the crimp die and after? Measure near the mouth where the bullet is located in the case. This will also indicate if the case is being sized down by the FCD. Food for thought.
Shooters. We are a community. United we stand divided we fall.