I’d like the devs to say something official about it rather than have guesses. A 250 mb save file doesn’t just restrict the number of saves for SSD users – churning that size file is going to cause serious drive wear if the game’s autosaving 250 mb every year of in-game time.
If it’s 250 mb that suggests the entire terrain elevation data is being saved in each file. If that’s the case then splitting that into a base generated mesh, deltas and other save data might not be a trivial task that they’re going to achieve in the next two weeks.