With regards to compression chips; If someone can substantiate this vague recollection I have it would be great.
I seem to recall Nintendo charged a per-unit royalty based off of the ROM size of the cartridge, which would make including a dedicated decompression chip reasonable, particularly since they are likely fairly simple to design anyways.
[edit] In addition, the two games listed as using the S-DD1 were 32M,it and 48M,it roms (there were very few 48 Mbit roms and 32Mbit was considered "large"), so these were already fairly expensive cartidges.
Found a pic of the Star Ocean PCB; note that the S-DD1 takes up less PCB space then even 1 of the two ROM chips:
I seem to recall Nintendo charged a per-unit royalty based off of the ROM size of the cartridge, which would make including a dedicated decompression chip reasonable, particularly since they are likely fairly simple to design anyways.
[edit] In addition, the two games listed as using the S-DD1 were 32M,it and 48M,it roms (there were very few 48 Mbit roms and 32Mbit was considered "large"), so these were already fairly expensive cartidges.
Found a pic of the Star Ocean PCB; note that the S-DD1 takes up less PCB space then even 1 of the two ROM chips:
https://snescentral.com/pcbboards.php?chip=SHVC-LN3B-01
[edit2] The SF Alpha PCB was much simpler by comparison:
https://snescentral.com/pcbboards.php?chip=SHVC-1N0N-01