storage saying “system” full

I gave my old 6s to my dad yesterday and it is saying he is using 62.91GB of 64 under “system”. He has no photos or apps to speak of so I don’t know what is under “system”. I’ve read about data issues and viruses using all the space. I have done 3 back ups and resets to no avail. It wasn’t like this whenI used it a week ago or when he used his old phone (a 5s). Please help!