I went to install some software today (Gradia) which is only available via Flathub apparently, the install size is larger than my distro which feels like a bit much for a screenshot tool. What’s the deal with the massive size of Flatpaks and is it really worth all the extra space? 4GB per application?

Have you ever tried to install a package and the install failed because of a missing dependency? You spend time chasing down that specific dependency, you find it and try to install it, but then the dependency itself fails to install because of a further missing dependency of its own. This is two steps into “dependency hell”.
This doesn’t occur with a Flatpak install because all the dependencies are baked into the Flakpak.
I am going to consider myself lucky as I have never run into that in 15 years.
The dependencies aren’t “baked into” the Flatpaks in the sense that they included in their archive, like e.g. AppImage, but Flatpak manages to install several different versions of depencies.
Thank you, that hasn’t happened to me in some time, but I understand how Flatpaks avoid that now.