The way you collect the folder sizes is most likely wrong, see https://forums.docker.com/t/huge-increase-in-lxc-backup/150886/3.
Furthermore, I can not tell whether Portainer actually shows the real image size. I don’t use Portainer, I feel it’s cumbersome to use, and I am not patient enough for “clickOps”. The containerd image store holds a compressed and an uncompressed copy of the image, not sure if Portainer “knows” this.
Note: this is true for fresh Docker Engine v29+ installations. Existing installations that update to v29+ don’t use the containerd image store, unless you configure it specifically use it! Since most of your data is located in /var/lib/containerd you must be using the containerd image store.