You might have some old, out-of-date backups.
First, do a backup of what you currently have, to be safe.
In iTunes: Preferences, then Devices tab. There will be a list of the backups. Might be able to see if one or more are "oddballs" (eg. I had three backups from my old 4S). Can delete from there.
All else fails, can drop into the individual backups and view the Info.plist files. Look for the string "Product Type" and the entries that follow: will tell you device and OS that backup is related to and give you a hint to what might be safely deleted.