Sorry for the additional post, but the title of this post doesn’t really reflect the issue and I am not able to change it. Therefore linking from a better title to the post here.
Hi tomoqv,
flatpak follows the Container like or static link approach and needed dependencies got installed by the flatpack-app too. I may be wrong cause i take just a quick look.
You installed like 5 Desktop Environments (two Freedesktop Platform, KDE, Gnome 41 and Gnome 42) parallel, which is ok on a Desktop Computer with enough space, but a bad Idea on a Phone with small space.
And Gnome got installed twice, (Version 41 and Version 42) cause some packages are so new that they developed on the 42 Version and others are not ported yet. And Flatpack installed you Gnome twice… 
You can use flatpak and install one package you need after another and check your lost Disk Space after installation. If one pull another new DE, just remove it and the DE again.
Its because KDE and Gnome use different Methods and Tool Kits to render Windows, Buttons, Menues… and other stuff. KDE use QT and Gnome GTK. Since Purism use Phone Shell Gnome try to start with Gnome or GTK Apps, or enhance your Disk Space.
Here are some Pictures and Howto’s to bookmark for later. I drop this Link before, cause parts of it was already explained in your Disk Space thread.
Have Fun!
Chris
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@Christal Thanks for the info, I’ll study it! I have just uninstalled and purged all flatpak related stuff on the phone and now reinstalling one app at a time checking disk space afterwards.
Just curious, are you using sudo flatpak install?
Mostly, yes.