The SNES CD has 256k of ram, according to Byuu, so they would have had to load in parts of the game. From Byuu's tests, maybe only 128k was planned. http://forums.nesdev.com/viewtopic.php?f=12&t=13907
I'm guessing the SNES copiers actually had 16, 32, and later 48 megabit of ram in them.