From Forbes:
"That 5.8-inch display that’s billed as the largest on an iPhone? There’s actually a bit of marketing speak at work there. The iPhone X uses a narrower aspect ratio than the iPhone 8 Plus. This means letterbox pillars when watching 16:9 HD movies (or going full screen and living with the notch cut into the display by the TrueDepth camera). But the narrow aspect ratio means the 5.8-inch iPhone X display (which is measured diagonally) actually has a smaller area than the 5.5-inch iPhone 8 Plus.
Including the notch, the iPhone X offers 12.97 square inches of display. Take the notch out and that becomes 12.49 square inches. Some will argue it’s actually a bit smaller than those numbers because of the display’s rounded corners. The iPhone 8 Plus display is 13.04 square inches. Again, not a big difference, but take that iPhone X “biggest display” claim with a grain of salt."