I want to install the game and on my "Games" page the Windows installer says it is 15.2GB. Then on the "extras" area it says that is 9.9 GB of extra content. Is the 9.9GB of extras within the 15.2GB of the game, or an additional file for a total of 25.1GB?

Is there a way to install the game without the extra content?

The extras are not needed to install or play the game. They are also not included in the installer download. They are literally "extras"; stuff that you might find interesting or useful but not needed for the game to function.

thanks - so just the game is 15GB?

Öh, well strictly speaking no, as it also bundles the different languages and the Redkit and what have you. But if you want to actually install the game, that's what you have to download. I think it was some gigabytes less some time ago (before Redkit release), but it's now all bundled together.

that's a big ass download