@phoneboy In August I demonstrated how to set up a new iOS device from an iTunes backup. The device used was a 128 GB iPod Touch with 47% of its space used. I backed it up with iTunes 12.7 then after the demo was over, rest it to backup to iCloud.
Two weeks later I observed that I had under 4 GB of free space in iCloud, it should have been closer to 25 GB free. The backup of the iPod Touch had grown to 21 GB. I assume most of that was apps.