If it takes too long on some devices, just add a setting to enable or disable autosaving. Also, I think, from experience, that the file size shouldn't change regardless of how much data you have to store (just block out an entire chunk whether or not there's relevant data). I think it was because that is easier to debug, but it also makes sense if you think about the storage (no fragmentation). Does the smaller size matter that much (I think that a The Last of Us save file is 10MB)?