So, it's not a huge thing, but have you considered having your save files be gzip-compressed json instead of raw json? I just tried compressing one of my saves (using WinZip, but same idea) on a whim, and it went from 900KB to 70KB. I know both save files are pretty small in absolute terms, but I have a habit of making a new save file on each save in case I mess something up, so it adds up.
If you're worried about save backwards-compatibility, you can save the new files as [name].aut or something, and still parse [name].txt files as raw json. (And maybe have an option to save in the uncompressed format for people who want to fiddle with the save files.)