Have the same issue here. I'm at 27 GB "Other System Data" and it keeps increasing the more photos, mails, apps etc. I delete. Cannot use most apps anymore due to full storage. Completely erasing is not an option, because I don't want to go through the lengthy registration process again for all the services I use my device as authentification for. Turning off everything iCloud-related didn't help. Seriously considering to upgrade to a device from a different manufacturer, if that doesn't get fixed.