no, flatpak has deduplication, so it can take less, and less space, from more flatpak you install(the first install is heavy, but it's downloading the runtime that others flatpak also share with)
If you only have 2 or 3 flatpaks installed it's going to eat space like crazy. Deduplication only matters when many are used and if they have the same base. I've already suffered from too much space occupied because of flatpaks.
But the tradeoff is useful for bad software like this, at least it works, if you have space enough for it.
It looks like you're right. Uninstalling the deb and then installing the flatpak consumed an additional 2GB on my root, but I have a handful of other electron-based apps that are mildly obnoxious snaps, and migrating them might help amortize that cost