Disclaimer
Flatpak uses OSTree, like Fedora Atomic Desktops (Silverblue, Kinoite etc) and similar to BTRFS snapshots.
So many files are deduplicated and linked, not actually there
https://gitlab.com/TheEvilSkeleton/flatpak-dedup-checker
50GB without
31GB with deduplication
21,4GB with BTRFS compression
Flatlack is weird. Sometimes it’s really good, but then other times depending on what you install it really bloons up.
Those are unmaintained apps and you probably shouldnt use them. Poorly this is not as obvious and cant be enforced.