One thing that annoys me about backing up VM disk images, is that the size is based on how big the image is. This isn't just a proxmox annoyance - ESXi does it too. It would be nice if vzdump could use something like clonezilla to copy the filesystems within the disk. Obviously not clonezilla itself, since that is a "distro", but it uses things like partclone and such to copy partitions to images...