GOG Account page says the game download is 473 MB but GOG Galaxy attempts to download 1,7 GB. Where's the difference coming from ? Do I need to download something after installing from the 473 MB installer ?

I think you're just looking at two different downloads for the same thing. If you're installing the game via Galaxy, I think it's probably just grabbing the installed files from available p2p connections, whereas if you grab the 473MB installer directly from the GOG website then you're getting the same thing as a compressed archive that installs to the same size in the end.

The download is 635MB for the Linux version (which also contains executables for all other OSes) which, when unpacked, is 1,7GB. So no, you don't need to download anything after you have installed it (they just have some pretty good compression in place to minimise your downloads).

Ok, thanks. I'm on limited bandwidth and it's vital information for me.