Note perso:
Backup iCloud

• Backup dit 3 Go
• Pourtant plus grosse app même pas 100 Mo
• Messages désactivés sur iCloud (0Ko)
• Aller dans stockage iPhone -> messages
• Supprimer les photos et vidéos (à la main …)
• La sauvegarde sera plus petite
• Il est peut-être nécessaire de supprimer et recréer la sauvegarde
• Il est peut-être nécessaire de se reconnecter à iCloud

Ici:

Like Matthew Greenwell I found that the majority of my backup was actually taken up by my messages. Even though messages was disabled under "Apps Using iCloud" this was still happening and my backup size was ~4.8GB. The fix for me was to actually enable "Messages" under "Apps Using iCloud", then promptly disable it again. Next I had to go into "Manage Storage" select "Messages" and then click "Delete & Disable." This tricked iCloud into recognizing the actual size of my backup which is now down around 180MB!

Seems like this is a bug in the iCloud management code. Anyway, hope this helps someone else.