It's not opening up in full screen (and there's nothing in the settings to change that), and it's changing my resolution to be way smaller. It's distracting to play a game that's taking 3/4 of the screen and what you can see of your desktop is huge...

Any way to fix this?

try alt-enter

https://steamcommunity.com/app/544970/discussions/0/2592234299549858172/

For me the only thing that helped was to change it to windowed mode (by Alt+Enter after launch), then double-click the window bar for the window to fit the screen and then Alt+Enter again. Then it was in glorious 1080p. Hope that helps :)