As several others have posted, the app does not support higher resolutions. For example, I upgrade from an S8 to a OnePlus 7 Pro and now I am forced to constantly try to resize the playback. I would really like to keep my phone resolution in Auto Switch, and not always on lower resolution FHD+/2336x1080. My OnePlus 7 Pro handles QHD+ / 3120x1440.
I feel like this is a usability issues, although probably not a huge pain for most at the moment. In time I know this will affect more and more users as they move to the high res phones on the market.