Why is my iCloud storage space showing full without any files?

Have you been using the Shared iCloud Photos Library? When you joined or set up a Shared iCloud Photos Library, it may have cached a backup copy of your original library, so you can revert to your original library. This backup will use your iCloud Storage, even if your Photos Library is empty for unto 90 days. This is documented in the Photos user Guide for Ventura, but no longer in the user guide for the more recent system versions, see: Create or join an iCloud Shared Photo Library in Photos on Mac - Apple Support (IN)

"When you join a Shared Library, iCloud captures a one-time status of your photo library database and metadata and stores it for up to 90 days. This helps with a smooth transition to iCloud Shared Photo Library. Any photos that were in your library at the time of the snapshot that are subsequently deleted are stored for up to 90 days."

I had to wait for ninety days before I could downgrade my iCloud Storage for my second Apple ID, after I erased my iCloud Photos Library for that account. So long did it take until the snapshot of my iCloud Photos Library had been erased.