Xtend uses aspect ratio scaling by default, meaning at any window size, a central area will be preserved that renders the original viewport. You probably don't want the entire room to be that central area—the room should never be used to set resolution.
I would need to know more details about your setup to say for sure, but it sounds like with your game setup, the default Xtend config is hitting the limit of what it can do on your display, and compensating for it the best it can.
I would recommend setting up your own viewport in your room settings and then customizing xtend_config appropriately for your project. The documentation will walk you through each available setting, and can be found in your project's Included Files area or online at https://docs.xgasoft.com/xtend