I only downloaded the game once;using the GOG downloader. I thought things were taking a bit long as it took FOUR HOURS at high-speed connection. I checked disk space and found out it took up almost ELEVEN GIG of room,when it was only supposed to take up 3.6 GIG. I did not install or download anything else before or after downloading or installing Myst 5. Now I am worried that somehow something "nefarious" was installed. My Norton AV detected nothing out of the ordinary,and only shows the Myst 5 being d-loaded and/or installed. WHat the heck happened?
I always double check prior to installing games to see how much room my HD has left (and I am slowly running out of space)....from 34 gig to 23 gig is 11 GB...the game was only supposed ot be 3.6 GIG.
Anyone else has this happen to them with a GOG download?

The download is 3.6GB, if you haven't deleted the install files that space is still taken. Then there's the size of the actual install, wich should be larger than 3.6GB since there's always some knd of compression going on with the install files, so the actuall game folder is likely taking another 5GB or more of HD space, so check the size of the install folder. You may also want to check the folder where the downloader stores the install files, the downloader sometimes doesn't clean after itself properly which means that in adition to the install files you can end up with partial files that should have been deleted after the merge - if there are any you can delete them manually.

For instance, the download for The Witcher 2 EE is 15.6GB. The install folder is actually 22.5GB, so between the install files and the actuall instalation of the game i'm looking at around 38GB of HD space - and that's not counting 2 or 3 GB's worth of saves saves since those are stored outside the game folder.

Thanks for the tips/ THe Game is runniing fine. And when I check the d-l folder,it only shows the 2 parts of the install.
I am still positive,though, that the game used 11 gig of space to install this game,and I cant figure this out.

Why don't you check the size of the game folder to see how much HD space it's actually taking ?

It's in C:\Program Files\GOG.com\Myst V