iPhone 16gb 5s shows 15gb of "System" whilst an identical iPhone using more apps shows 7.98gb of "System"

My wife uses her iPhone 5s in a simple way, no internet use and only a few apps but she keeps getting iPhone storage full messages. Her phone shows 15gb of System. She is on IOS11.4.1 and cannot update to IOS 12.1.12 due to lack of space. "About" shows 0 songs, 0 videos,11 photos, 2 applications, capacity 16gb, available 327.7MB

My identical phone has loads more apps on and I use the internet quite a lot and my phone shows only 7.98gb of System and is on IOS 12.1. "About" shows 2 songs, 36 videos, 867 photos, 42 applications, capacity 16gb, available 3.03gb.

Does anyone have any idea as to what is going on with my wife's phone as I'm at my wits end trying to work out what is using about 7gb of system memory??? Both phones are about 3.5 years old

My sincere thanks for any help

David