Hello,
when I downloaded installer on the day 1, it was only around 6 GBs. Now today, the installer has been updated with one which is almost 9 GB large.

What gives?

A change log would be fine. They updated their version from 2.2.0 to 2.3.0. What has changed? Is it the same upstream version (1.0.57)? Do I need to reinstall, do I miss something if I don't. GOG, there is no excuse for not having a change log for your own stuff.

2.3.0 Installer is game version 1.0.57.0
You don't need to apply the patch 1.0.57 after installing it.

I'm unsure why it's 2Gb bigger than day one game.

Additional note.
If you installed patch 1.0.47, then patch 1.057, it can happen your game crashes on quickload. Happened to me.
The solution is to uninstall the game, download this new release and install it (make sure you choose to keep savegames on uninstall).