Dear Maintainer, when I purged the flatpak package, /var/lib/flatpak and all of it's contents are still left here — 21 MB, quite a lot.
Just installed flatpak to quickly try out an app and after purging I had over 2GiB left in /var/lib/flatpak. Anything which is system-installed and can be reinstalled should be purged when a package is purged. Only user-generated content should be kept (although there's an argument that the user should be prompted to remove any personal settings files as well).