Not enough iCloud storage available?

I don't get it. My phone says it won't backup because I don't have enough iCloud storage available but I have the 5GB available out of the 5GB it came with.

It's an iPhone 7 and I have used 40 GB and have 70GB left and that seems like a fine ratio to me?

Wait what? You have used 40gb and you're wondering why you can't back it up to iCloud which has 5gb free? 40gb is bigger than 5gb, if you have a box that can hold only 5 chocolates, then you can't store 40 chocolates in that box.

Pretty simple, you have a 40 gallons, to pour into a 5 gallon bucket, how do you make that 40 gallons fit into just 5 gallons? The answer is you don't. So what needs to happen is you pick and choose what gets backed up to the iCloud space, makung sure its 5GBs or less, and back everything else to your computer with iTunes.

The basic backup looks at EVERYTHING and calculates the total and then compares it to the space that is available.

Only back up 5g or less of CRITICAL stuff to the cloud. Photos and other things can get backed up free at other places like Dropbox, Google, and other sites. Look around. And if your music and photos are already safe at home, you do NOT have to back them up again! Clean out your life a little.