It will install in the same directory as your existing game files.
I installed Summerset last week, and while it does seem as though it's going to do a full install, mine took just seconds to complete.
Whatever it's installing, it is actually installing over 50gb and will take a lot longer than seconds. More like over an hour.
It just hit me that you may have purchased the full version of Summerset, instead of just the upgrade, in which case it would install the full game.
I just got Summerset, so I wanted to install it. I wasn't sure if I had the base game installed, but it's in my program files so I ran it. But it just comes up with an 'install' button in the window, so I clicked it. It began installing (the whole game? Summerset? I'm not sure) but it didn't give me any chance to specify where to install...