Backup taking up 2x as much space as it should

I'm running 360 v5.1 and having it back up  files. I have about 220 gb on my  hard drive and its backing it up to a NAS drive. I noticed last weekend that the backup file is now taking up twice as much space, about 440 mb. There is still just the one N360_BACKUP folder with all the oddly incripted files. Any ideas what the problem might be?