Pay for ICloud storage but still have storage taken on photo app

I pay for ICloud to backup my photos which it does but my iPhone storage keeps going down but when I look in the phone storage area it says my photos app says it 3.72GB it keeps going up. Why and how do I get rid of this as they are all backed up to iCloud and optimised as well. It says it’s for documents and data but this should be on my iCloud. Please help get rid of it.

[Edited by Moderator]