As a server administrator, I would like the backup system to create and upload a single backup at a time, preventing my server's disk space from filling up while waiting for the backups to complete and upload.
It appears that the new backup system packs (tar.gz) ALL selected accounts then uploads it to the remote FTP location. The local copy is then deleted from the server once the upload complete.
This approach requires a large amount of local disk space and the primary reason I am using remote FTP is to save local disk space.
This behavior has been confirmed by Cpanel staff (ticket #4413741).
I think a better approach would be to pack > upload > delete just one account at a time instead of packing all accounts first.