iOS 11 Takes Tons of Storage
Pretty simple triage - the “system” either now contains an iOS update cached or a lot of user data, but you can convince yourself:


Backup the device
Erase all content and settings


Measure the space. If you feel it’s still taking a lot of space, put the device in recovery mode and restore it.

Once you’re OK with the space usage of the bare system, erase one more time and restore from your backup of choice or just sign in and set it up new if you want to start fresh.