Apple - iCloud storage full: "Not Enough Storage". Backups exceed 5GB

What to do when you do not have enough storage because your backup size is too large:

• Delete the current backup on your iCloud for that specific iOS device.
• Your iCloud backup is now automatically turned off, so turn it back on.
• A new backup is made and has a reasonable size.

Theoretically your phone could crash in the short while you do not have a backup, so you could run an iTunes backup first.

My answer is based on My personal experience, so this may or may not be valid for you.

Based on the information provided by you, it reminded me of a similar situation I had face sometime back. iCloud not back's up the recent backup data but also saves the backup files backed up before, As in, If I created a backup today, and then I created a backup next week, it's going to keep both the data as I might want to go to the lower backed up version in case you want to restore from not the most recent version of the backup. It looks something like the image below.

That's what is clogging the space in your backup.

P.S: This Link will explain how to restore to the not so recent backup proving my point as well.

The solution that worked for me tonight on an iPhone 7 running iOS 12.2 which was showing a "next backup size" of over 8 GB despite almost nothing on the phone being selected for inclusion in the iCloud backup was to change the "Keep Messages" duration at Settings > Messages from "Forever" to "1 Year".

(This was despite Messages set to not use iCloud at Settings > [My Name] > Messages, in the "APPS USING ICLOUD" section of that page.)

Doing this reduced the "next backup size" (as reported at Settings > [My Name] > Manage Storage > Backups > This iPhone > Backup Size to 3.6 GB, and allowed backup to iCloud (using my free 5 GB storage) to succeed.

I also noted that Messages didn't appear at all in the list of apps at Settings > [My Name] > Manage Storage > Backups > This iPhone > Show All Apps. Evidently, Messages data (including photos and videos sent and received?) is always included in iCloud backups?