Q: iPhone 6 Whatsapp iCloud Backup (extra storage)

Hi,

I recently tried to backup whatsapp on my iPhone on Apple iCloud. But whenever I am trying to do so (on whatsapp: settings > Chats > Chat Backup > Back Up Now), my phone gets an error message saying that I need more storage on my phone. (I have approx. 800MB free storage on my phone / 4.9 GB free storage on iCloud)

But I just want to backup my whatsapp files on iCloud storage, not on my local phone storage (just 6GB in total). How do I fix that problem?

It also looks like that the whatsapp backup feature is saving all its data (backup files) on my local phone storage, but not into iCloud. That doesn't seem logical for me at all. Can anyone explain me how this works?

Thanks,

Edit: I have now successfully backuped my whatsapp files, but my phone says that I have used 904,3 MB on my iCloud storage but also that I have only 269 MB left on my local storage (before the backup I had approx. 800 MB free storage on my phone).

Does Whatsapp save it's files on both storages? On iCloud + local iPhone storage??
How can I change that?