This issue occurs often because the backup doesn't immediately update to reflect deletions, causing the backup size to remain inflated temporarily. Here's what's happening and how you can fix it:
When you add large videos or files to your phone, your iCloud backup naturally increases in size. However, when you delete these large files, the backup doesn't always instantly recognize that change, leaving the displayed backup size unchanged at the larger value.
To fix this, you need to refresh or force-update your iCloud backup. A simple and safe way is to first ensure the videos are deleted permanently from the Recently Deleted folder within your Photos app, since deleted videos often remain there for up to 30 days. After you confirm they're completely removed, go into your iCloud settings (Settings > Your Name > iCloud > Manage Account Storage > Backups > [Your device name]) and check your backup details to confirm that Photos or Videos aren’t listed as large items.
Next, restart your iPhone. Hold the side button and either volume button, turn your phone off, and then turn it back on after about 30 seconds. Restarting helps your device accurately reflect storage changes. After restarting, manually perform an iCloud backup again (Settings > Your Name > iCloud > iCloud Backup > Back Up Now).
Doing this forces iCloud to refresh and recalculate the actual backup size, removing data from deleted files.
Usually, after following these steps, the backup size returns to its correct, smaller value (close to the original 9 GB).
If your backup still shows the inflated size after these steps, it could be a delay in Apple's servers updating. Waiting several hours (up to a day) typically resolves the problem automatically.