How to reduce flatpak app size?

[Mike Spadaru]

Dear Flatpak community,

I attempted a small experiment to benchmark Snap and Flatpak. On
Ubuntu 16.04 I picked some apps: Spotify, VSCode, Telegram-Desktop,
Gimp, Android-Studio. I performed three tests Native (via apt, .deb or
PPA), Snap and Flatpak. In each test I would read the used disk space,
install an app, run the app and perform a task (same task in each
test), then read the used disk space again. Before moving to the next
test, I uninstalled all the apps and removed any config files created.

The machine uses only one disk partition. I also recorded CPU, RAM and
Power consumption, but there was no clear difference between the three
package formats.  My results for the disk usage are shown bellow.
Values are in mega-bytes:

| App | Native | Snap | Flatpak |
| Spotify | 240 | 290 | 980 |
| VSCode | 270 | 220 | 1030 |
| Telegram | 80 | 100 | 600 |
| Gimp | 80 | 200 | 450 |
| Android Studio | 3290 | 3330 | 5930 |

I started this experiment because I wanted to investigate package
formats that I could use on a Chromebook running Linux. The specs of
this machine are not great and it only has 16 GB of built in storage,
so performance and disk space are crucial. I could not find any
existing performance comparison of apps using different package
formats. While I would much prefer using Flatpak due to what I
perceived as a better user experience, I do not have sufficient disk
space on this machine.

Each Flatpak app uses considerably more disk space than Snap and
Native. I understand that they need to install runtimes, but is there
any way of forcing the apps to share runtimes? Or is there maybe any
other way I could minimize the disk space used by each app?

Cheers,
Mike