SUMMARY Discover in the app info shows now the size of package itself and the number of dependencies. I think, it will be more useful to calculate the total size of all packages, that will be installed, if I choose to install that app.
I agree. Even if there's no exact way to know the size of an *installed* app due to shared dependencies, there is indeed a way to know the size of an app plus the new dependencies that need to be installed, which answers the question, "how much extra space will be taken up if I install this thing?" The answer is, "The size of the app package plus the size of the new dependencies that need to be installed"
This needs implementing on a per-backend basis. Here's the bug for Flatpak: Bug 402636. Moving this to PackageKit.