I create a brand new App with the Dimension of 1920x1080 and turn off Scale to Fit. Then add some labels that show the values of the App.Width/Height and App.DesignWidth/DesignHeight and play the app in a web player on my monitor that is 1920x1080, it reports that the app is:
As a result, everything on the screen looks incredibly small in comparrison to the designer (at 100% zoom) as the pixel density appears to be way too high for some reason.
Edit: I checked the values within the app on iPhone and they all look as expected. But if I look at it with the App from the Microsoft Store the same issue occurs that im experiencing in using a web browser.