as others pointed out, the X1X has 12GB, not 16GB.
and i will add that you shouldnt be comparing to the X1X increase becasue it released only 2 years ago, but also GDDR6 vs GDDR5.
and then i will also add the advantages that very fast SSD will do to the RAM, first off, because you can swap data in and out so much faster, you will have much more "frame relevant data in the memory, for example, at a certain current gen frame, you actually have quite a lot of excess memory that is not even used in this frame, that just the HDD isnt quick enough to swap out and swap in more data for this frame, with the very high speeds of the SSDs, you can suddenly load a lot more data per frame to make sure you are using your 16GB to its fullest.