iOS Photo No Longer Optimising Storage
This happens on occasion. For some reason Photos stops removing full-resolution versions of photos. If your iCloud Photo Library is up to date, go into Settings / Photos &amp; Camera and disable iCloud Photo Library.

This process may take a long time, and will leave all your full-resolution downloads on your device.

You can open the Photos app to see the number of photos decrease. Don't do anything until it's stabilized. Once it has, wait a few more minutes just to be sure, then delete all remaining photos. Remove them from Recently Deleted as well.

You can now reboot and turn iCloud Photo Library back on with Optimize Storage enabled.