iPhone 8+
1920-by-1080-pixel resolution at 401 ppi
screen width = 1080/401 = 2.69 inches
iPhone x
2436-by-1125-pixel resolution at 458 ppi
screen width = 1125/458 = 2.45 inches
The 8plus screen is wider by essentially 1/4 of an inch.
Excellent question. I had the same one. Everyone is so concerned with the diagonal measurement. The actual display width is by far the more important one to me. I've done the same comparison with the iPhone 8plus and 12 and 12 PM.