Did we not have a discussion on here a couple of years ago about the Irish VLCC ' gaining advantage ' by registering some / all its fleet with lower gross weights in order to benefit from lower En Route Charges from Eurocontrol ?
There is another European LCC who does/did this. Not sure about a previous discussion, but they registered lower Max weights for the fleet. The weight of the aircraft was what it weighed but the max payload was reduced. From memory, the structural limit was around 71T, one group reduced to around 64T, another to around 62.5T and some more to around 60.5T. No idea of the saving it generated but with large fleets, even small amounts will help and go straight to the bottom line.