Why can't I find out what is supposedly using all of my account storage on icloud?

My Iphone keeps telling me that it can't complete a backup because my storage is full. I have nothing particularly big which should be using up masses of storage saved to the phone. When I click 'manage account storage' it just says 'backups 5GB'. However, when I tap on the 'backups' icon, it simply says 'documents and data 1.7gb' and iphone this phone 1.7gb'.

I've deleted most of the big videos etc... from my phone and shouldn't have anything that should cause it to be anywhere near 5GB, but nowhere I can find will it tell me what is actually taking up all this space or offer me the option to delete anything. It does, however, keep trying to get me to sign up for an upgraded account, which I have no need for, I just want to know what is taking up so much space and be able to delete what I don't need. This is really frustrating, any help appreciated.