If you’re one of those people who fills very large levels with many materials ( hence textures ), and grass and trees and… And you were already using a healthy amount of VRAM, I think moving to SM6 would push you over.
It’s no good, can’t find anything that really nails it. All I know, is the DX12/SM6 switch had a major effect on shader instructions and complexity, and I’m assuming that is getting translated to VRAM.