I had the same issue even with 128gb iPhone 6 Plus, just like everyone here storage capacity was 119GB and apparently i used up everything and elft with 0 bytes. What was worse was that I couldn't backup my iPhone because it just says it couldn't back up because iPhone was disconnected (when my cable was showing that iPhone is charging and I could access the files in my iPhone while it was backing up).
Two solutions:
1) Fix the backup issue first then Back Up Now
- Delete previous backup files as it could be corrupted. How to? Menu (top left corner) / Preferences / Devices / Delete Backup
- Now click 'Back Up Now'. Back Up could take up to an hour or so. Mine took almost 35 minutes. However at my first attempt it was successful.
2) To fix storage full issue.
- After you performed the Back Up, click Restore iPhone.
After resetting the iPhone, you can now observe on your iTunes that your storage has free up a lot more space by now at the indicator located at the bottom beside the Sync icon. in my case, I now have 98.20 GB free from 0 bytes free.
- Restore your iPhone using the back up file and if you have chosen Encrypt iPhone backup previously, a pop up will now prompt you to enter password and this would be the password you have entered for the Encrypt iPhone backup. Initially i thought it was iTunes password or my phone's unlock password. After a number of attempts i remember i had an Encrypt iPhone backup password. So don't panic.
Result:
Although i lost all my WhatsApp images that was saved within WhatsApp, however i managed to restore everything else including my WhatsApp chats. Free up 98GB worth of space from supposedly 0 bytes. Believe it or not, my phone now works a lot faster and hangs less now.
I hope this helps.