So I started downloading TW3 earlier, and galaxy told me the download was 20+ GB. Ok, went to do other things for a while. I come back, and the download has added another 20+ GB, and the total download size is now up to 50GB. What the hell? Is this normal?

I believe the size will roughly double after it's unpacked (original plus slightly bigger unpacked data).

I'm not talking about the size of the folder...it's actually downloading a total of 50GB worth of data. If it's not some sort of duplicate, this means the actual installation will be insanely huge.

When using the Galaxy client the size of the file shown by the Galaxy client is for the unpacked file size, when you install a game using Galaxy part of the installation process is unpacking the game files & installing the game automatically for you.

This is why the Witcher 3 download is shown as 20GB but when you use Galaxy to install the Witcher 3 it shows the games actual file which is about 35GB size when fully installed.

On the left hand side of the Galaxy screen underneath the list of games you have installed there is an arrow pointing down, click on this arrow & you will reveal your download queue showing you exactly what Galaxy is downloading