There is a slight confusion in my mind, I hear that the ground cable must be at least one size down from the battery cables and that the same applies to the inverter ground cable.
Victron Wiring Unlimited clearly specifies that the Grounding cable must be able to carry at least the same current as the battery fuse rating (I’m compliant with that) but does not specify the size of the inverter ground cable. Also, the inverter ground connection does not offer many options over a certain size because it only carries a 6mm stud/nut hence not indicating the need for large lugs/wire