I'm downloading SR2 and the store page says it's 5,8 GB but the download says 13 GB. What ?
How are you downloading the game? The store page will be for the GOG installer, which will likely have compression in place. But if you are downloading via GOG Galaxy then that downloads game files directly, so the compression may be different (if there even is any). Note that the game page also says it needs 15GB hard drive space, so that would be the required space for the install (and possibly also any swap space), which is more like the download size you are getting.