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
Yes, Appimages lost the race. The apps are old afaik, and often dont work. It sucks when distributors use Appimages as they are simply a bad app format.
There is a way to convert Appimages to Flatpaks, but I havent got that complete.
https://github.com/trytomakeyouprivate/Appimage-To-Flatpak