This is an optimization issue. I agree the plus screen is slightly and I mean slightly bigger, but the reason things are cut off is because the video is not made to fit the aspect ratio of the iPhone X. If and when things are filmed to fit this display, this won’t be the case anymore.
I'm not expecting the world to move away from 16:9 anytime soon. I think at best we will start seeing some 18:9 next year, but on a whole 16:9 is incredibly entrenched and will not be be moved easily.
However, for movies, the X is awesome. Get a good demo for X as a movie viewer in full screen with this: