Tap that quadrant and you’ll see a breakdown of how much storage space you’re using.
I have tried the answers for the iPhone 4 explained in the question Why is so much 'Other' space occupied on an iPhone 4? (except for factory resetting.
I have an iPhone 6s with iOS 11 and I have just noticed that a large amount of my iPhone's storage space being occupied by 'System' (12.27 GB).