When you install a Flatpak, it will need runtimes (GNOME, KDE, Freedesktop...). These are the ones that really take up disk space. But the good news is that it looks big if you install just one Flatpak application, but the more you install, the more they'll use the same runtimes already installed, and the less disk space they'll occupy.
And no, ungoogled-chromium doesn't require 4-5 GB, just 153 MB...