After using microk8s for a while, there may be increased disk usage due to a number of docker images saved in /var dir. You can delete these images using crictl tool. It is not part of microk8s installation, so you need to download first.
Install crictl
Download crictl
Connect to containerd
sudo ./crictl -r unix:///var/snap/microk8s/common/run/containerd.sock image lis/
Delete unused images
sudo ./crictl -r unix:///var/snap/microk8s/common/run/containerd.sock rmi --prune