If you are space concious, you should try to select Flatpak apps that are using the same runtime (Freedesktop, GNOME or KDE), and make sure all of them are using exactly the same version of runtime. Correct me if I'm wrong, but only two versions of Flatpak runtimes are supported at a time - current and previous. So during times when transitioning happens to newer runtime, some application upgrades are not done at once, and user ends up using more than one (and sometimes more than two) runtimes. In addition to higher disk space usage, one must account for usual updates too. The more programs and runtimes you have, more updates to download. Good thing, at least updates are partial.