When they released iOS 11.3 it split messages from the backup and gave it its own section in iCloud even though igloos messages didn't ship. This resulted in the backups taking less space. A few days later the messages section disappeared but the general backup size didn't increase back to where it was. It still uses the same storage so backups will say 2gb used even if your backups only equal 1.5gb, the other .5gb will be messages. I hope that makes sense but that is what happened.