After adding Proxmox Backup Server 1.0-6 to the Proxmox VE 6.3-3 cluster, the storage size was incorrectly displayed.
The final size is too large due to the number of servers in the cluster, to each of which the Proxmox Backup Server is "connected".
FYI, a fix for this was applied in git and will become available with libpve-storage-perl >=6.3-6, but it's not packaged yet, so don't expect it right away.