On PC graphical assets are duplicated on system and vram. We have no way to go to IO and bypass memory at this moment and it would be too slow.Here I am reading the minimum specs on PC are 12GB total ram. But then XSS has only 10GB (and like a PC has an OS too). So technically XSS specs are even lower than the min PC specs. It's not vram limitations. It's main ram (making sense for this kind of game + splitscreen) + vram.
Its 8GB max.