What's on your computer is a phantom listing. The only space it uses is that needed for the entries.
That's only for files from apps that can save directly to the iCloud drive, Numbers, Pages, etc. If you add local files/folders to the iCloud Drive folder locally
the files just get moved to that folder which is in the ~Library/Mobile Documents folder and remain on the hard drive while also being copied to the iCloud Driver servers for access by other devices. However, those files must be downloaded to the other devices before they can be viewed or used on that device. They can't be viewed or otherwise accessed at iCloud.com.
So if one were to drag several folders of files into the iCloud Drive window as shown above all it's doing is moving them to a different location and copying them to the iCloud drive. There's no saving of space on the hard drive.