Temporary files are put in %AppData%\MedievalEngineers\CurrentSession, and are intended to be cleaned up once the files get moved to the final location during a save operation.
However while the files to get copied to the final location, the temporary files are not deleted until a new world is loaded.

This causes roughly the entire save worth of files to be leaked after each auto save, and not recovered until the session reloads. This is BAD but currently no data gets lost and fixing this is too dangerous to rush.
Until this is resolved ensure your game (servers) restart often enough that your host don't run out of disk space.