Flatpak download size

I added Flatpak support to my Ubuntu 18.04 and I'm confused about the package details it's showing me.

For example for GIMP package, it shows me it needs to download 1.4 GB in order to install 213 MB of the app.

For other applications (Android Studio) it wants download more than 5 GB just for one application. This is insane in my opinion.

Is this Flatpak / Ubuntu Software Center bug or just my misunderstanding?

You can always have a choice between official APT repositories and third-party ones and/or PPAs. For example GIMP has a PPA.

Snaps and Flatpaks need to download big runtime parts. You should learn more about them before actually using them. And you can choice - do you want to waste disk space and network traffic to use them, or not.

Some developer applications are available in Ubuntu Make.

In your particular case - GIMP needs to download GNOME runtime (runtime/org.gnome.Platform/x86_64/3.28) and dependencies (check flatpak org.gimp.GIMP.json for details):

$ flatpak install https://flathub.org/repo/appstream/org.gimp.GIMP.flatpakref
The application org.gimp.GIMP depends on runtimes from:
  https://dl.flathub.org/repo/
Configure this as new remote 'flathub' [y/n]: y
Required runtime for org.gimp.GIMP/x86_64/stable (runtime/org.gnome.Platform/x86_64/3.28) found in remote flathub
Do you want to install it? [y/n]: y
Installing in system:
org.gnome.Platform/x86_64/3.28             flathub 6d1d0ebbd724
org.freedesktop.Platform.ffmpeg/x86_64/1.6 flathub d757f762489e
org.gnome.Platform.Locale/x86_64/3.28      flathub 2823e3d81b74
org.gimp.GIMP/x86_64/stable                flathub 1fcb2c783cf6
  permissions: ipc, network, x11
  file access: /tmp, host, xdg-config/GIMP, xdg-config/gtk-3.0
  dbus access: org.gtk.vfs, org.gtk.vfs.*
  tags: stable
Is this ok [y/n]: n

Here there is one positive moment - this runtime may be reused by other later installed FlatPaks.

Personally I prefer PPAs over Snaps and Flatpaks and install several applications from umake (for example Arduino IDE).