Microsoft and Sony are reserving more SSD space for OS functions thus they don't need to rely on Ram as heavily...
Thus the Xbox Series X will only use 2.5GB of Ram which is a reduction over the Xbox One.
The Xbox Series S will use only 2GB of Ram which is a reduction again, likely due to the 1080P User Interface rather than 2160P.
Well thats an improvement, however 2.5gigs is still a fair chunk of Ram.
Imagine we get the slim interface of 360 and ps3 back. They ran at 1080p, less than 64MB for the OS, 32MB even I think at the end of the gen. 2.5 GB, what a waste!