I have an iPhone 6 and an iPad 4 mini linked to the same iCloud account, and have set synch settings in the usual way to backup photos and apps to the iCloud. I have storage plan of 50 GB, and currently using 37 GB from that. The app I use most the time on iPhone is Whatsapp.
The problem seems to be that iPhone local storage is running out, and I keep receiving warnings, even though the iCloud has plenty of space. I am not sure what to do to fix this. I checked all settings and tried to research it online, but still no answers. I suspected that the Whatsapp might be keeping the data locally (e.g. photos/videos), even though the iCloud has backed them up, but I am not sure.
Here are screen shots of the situation.
Best Answer
The problem is the local storage on the phone. The middle picture showing device backups isn't a problem - you have plenty of space in the cloud.
If you can't figure out how to prune what's app - you can try their support and/or deleting the app and letting the storage stabilize over a day.
Once you know how much free space of the 16 GB is available without over half being taken by WhatsApp - you can then document that and re-download that one app.
Things won't work well until you free up a GB of space and it looks like WhatsApp is the only possible solution to your current jam.