Screen is supposed to be bigger than the 8+, but if you take a photo with the phone it is much smaller than if taken with the plus. The phone shoul doa better job of re-sizing to an aspect ratio to take better advantage of the iPhone X screen size.
The display is a different aspect ratio than the 8+ (taller, but skinnier). You can’t alter the aspect ratio of a viewed photo without stretching it, or cropping it.
That’s the downside of the newer phones with this aspect ratio (iPhone X, Samsung S series and Note).