I've had the same problem.
My iPhone 7 32 Gb had a whopping 15.2 Gb taken by the iOS system. I fixed that resetting to factory settings by deleting everything. Then I set it up as a new iPhone and restored the last backup. Now the iOS system has 6.2 Gb.
Maybe iOS saves the previous versions so you can restore it... Or maybe is just a bug.
Did the same thing to my 2017 128 Gb iPad and the iOS system was down to 10.2 Gb (it was 31 Gb).
My iPhone 7 32 Gb had a whopping 15.2 Gb taken by the iOS system. I fixed that resetting to factory settings by deleting everything. Then I set it up as a new iPhone and restored the last backup. Now the iOS system has 6.2 Gb.
Maybe iOS saves the previous versions so you can restore it... Or maybe is just a bug.
Did the same thing to my 2017 128 Gb iPad and the iOS system was down to 10.2 Gb (it was 31 Gb).
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