I am now using a Samung S10+ and when watching football on the Optus Sport app there are thick black bars either side. Selecting full screen in the settings doesn't change the video. I assume it is to do with the screen ratio (19:9). Any chance there will be an update to allow full use of the screen in the future? With youtube you can pinch the screen to allow the video to take up the whole screen.
Are you using Chromecast?
We suggest logging this with via the Optus Sport app.