Exactly this. I remember Doom 3 on the original Xbox. It's a marvel of porting - running just 64mb of DDR RAM, they squeezed most of the game in to it. They did this by chopping it up, shrinking the levels, to fit inside that tiny memory footprint. If the open-world rumours are true, then I suspect we'll see loading screens and breakup on the XBone versions that simply isn't present on the Series X.