Can you clarify what exactly you backup? The whole file system of the lxc container? Just a specific path?
According to your output, it can not just be /var/lib/docker, as the folder size has 535M, which is smaller than the backup size of LXC debian 12 and LXC debian 13.
Can you share the output of this command?
sudo du --max-depth=1 --one-file-system --human-readable /var/lib