iPhone "Next Backup Size" incorrect / not able to backup to iCloud because of insufficient space - Incl. solution!

So I also had the issue that I could not backup to iCloud anymore because the next backup should be bigger than the remaining/overall space. This was especially strange since the sum of the below activated app-backups is not matching the "Next Backup Size".

The root cause is that these numbers of the individual app-backups can be wrong. (Please fix that asap, Apple!) For me it was Whatsapp which iOS showed as ~400kb to backup, while the truth was ~3GB.

How to find out which app/category backup is incorrectly shown?

• Go to Settings > Your Account > iCloud > Manage Storage > Backups > PhoneName/This iPhone
• If necessary "backup" your backup settings. So take a note or screenshot of the apps/categories you've got activated. (Hit "Show all Apps" before.)
• DISCLAIMER: The next steps have some risk. You need to make sure that you can go without the current backup on iCloud. If you are not sure, stop here and consult somebody who has experience.
• Deactivate all apps/categories.
• Scroll all the way down and "Delete Backup".
• Go back.
• Enter the Backup again. -> This should refresh the "Next Backup Size" and it should be 0 or only few kb.
• Now activate the apps/categories one by one and always go back and forth afterwards. (Otherwise the next backup size will not be refreshed.) With each app/category activated the Next Backup Size should increase according to the number of that app/category. If the increment is higher you've found (one of) your culprit(s).

Depending on the app or category you need to find your solution. Maybe you can clean up data from it? Or maybe you dont need it all especially if its overloading your iCloud? If its something like photos, videos or files, maybe back them up locally on a PC? etc. etc. In worst case, if you really need all that data backed up, you will have no other option than upgrading your iCloud.

For my case of Whatsapp it was quite easy since this discovery made me remember that Whatsapp is backing up itself via iCloud Drive, if activated. Activating Whatsapp as well in the normal iPhone iCloud backup would be a duplicate anyways, so I deactivated the latter. (You can also deactivate the first, pros and cons can be found on google.)

I hope this helps and Apple, please fix this display issue asap! Its probably a money maker, but leaving it unfixed would not be fair.

PS, maybe for the Devs: After deactivating Whatsapp I performed the iPhone backup. Not fully sure, but I think when that backup was finished the size of the not included Whatsapp backup was refreshed to the correct ~3GB.