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Hi Adamb1,
Welcome to the community,
This is completely normal for this device, nothing out of the ordinary.
Anytime, glad I I could help 
I have just updated from the Xperia Z2 to the Z5. The Z2, with Android 6.0, uses about 3GB for Android System. The Z5 uses almost 10GB. Even if that is normal, what are the extra 6GB or so used for? The features are the same as far as I can see.