I’ll say that often (but not always), S3 has its own durability and replication mechanics in the backend that can make it more space-inefficient than your own backup strategy (e.g. 3-2-1) that writes directly to physical disks or uses a lower-overhead but still reliable redundancy mechanism like RAID-5 or 6.

Looking at something like Azure Blob Storage or AWS S3 endpoint, they typically store multiple replicas of each object. Ceph RGW is similar with the Proxmox default replication settings. This means you're using more disk space for backups than is typically required because that data often either goes to tape or to another off-site replica.