MIK0: [...] They are usually pretty skillful in glossing over troublesome questions.
I don't see what's all that troublesome about the question I asked - since this was an internal update, i.e. a GOG process that has nothing to do with the actual game content, it shouldn't at all be hard to tell why that specific DLC lost 2/3 of its size in the process.
MarkoH01: Warning in case you do own Outlaws and Windows 7. With the latest update GOG decided to replace the windows 7 rainbow color fix for this game which so far woked great.
"replaced rainbow color fix with a less invasive method (Windows 7)"
At least for me this fix IS NOT working so I was glad to still have my backup of the old version of the game. So make sure the game colors are fine for you before you replace your old version of the game!
Thanks for the heads up, I downloaded this one specific for this part of the changelog. Good thing I still have my previous installer safely backed up.