Other possible causes for large uploads include torrenting/file sharing programs as well as cloud back up services such as Drop Box and iCloud.
Virus activity is another major cause of large uploads, so make sure that you update the security software on each device and run a scan to eliminate this as a possible cause.
If there is an email with a large attachment stuck in the outbox of your email program, it may be continually trying to send the email over and over.
I hope this helps you to track down the source of this data usage.
- Shelly