Zoid Kirsh on Twitter (while Elon Musk hasn’t completely wrecked it yet) tweeted about how Metroid Prime’s save system works. Metroid Prime save files are less than 60 bytes long! A single Gamecube memory card block is eight kilobytes, so it’s a bit overkill, but it’s still nice when a developer is frugal!
The way they explain it is that the game has a number of “world layers” which determine what is spawned in each area when it’s loaded. Which layer is active when a room is loaded is determined by a single bit in the save. That, plus some basic stats like health and ammo, and the record of object scans, all take up very little memory when bit-packed.
If Metroid Prime’s save file were 59 bytes long, that would mean it had 472 bits to work with. The passwords used by the original Metroid only stored 144 bits of data!