The 40CU PS5 die is rumoured to be around 310-320mm^2. It is unlikely those additional 16-20 CUs would take 90-100mm^2 (assuming a fifth gddr6 controler) in the XSX. It's still a guessing game in the end.
No, that's exactly how much it currently takes for an extra 16 CUs. Take the Radeon 5600 compared to the 5700. 16 extra CUs for exactly 90 - 100 mm2. The CU itself is smaller, but it isn't the only thing you need to add to increase a GPU in size.
Also do consider the PS4 Pro and the XB1X as an example since, despite the die shrink, the RDNA's features in 7 nm take around the same size as GCN on 14 nm. A mere 4 extra CUs on the XB1X for 25 mm2 or so.