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
TBH I dislike Appimage purely because I can’t be bothered to go and check them all individually for new versions all the time, it feels like being on Windows again. I don’t mind a little bloat for the sake of convenience. But that’s just personal preference of course.
There was an app that dealt with this but it’s since been abandoned.