Well, the size estimate on flathub assumes that you're installing every dependency, which only happens if it's the first app you're installing with this FreeDesktop version, which is rare. I have like 15 flatpak apps installed, all of which had a claimed install of over "1 GB", but the flatpak install directory is only like 2 or 3 GB.
There's just not a great way to predict how big an install will actually be from flathub.
Edit: just to give you an idea, since its only downloading the deltas, most of these "1 GB download size" Flatpak apps are downloading less than 100 MB