the plus models have the same aspect ratio as the regular models, which is 16:9
Ever since the iPhone 5, iPhones all had a 16:9 screen, before that it was a 3:2 ratio.
The iPhone X is the first iPhone with a screen wider than that, which does introduce pillarboxing (black bars on the side) for 16:9 content (your average youtube content or the movies you shoot with your phone.
Subsequently it's a smaller image than what you get on the Plus, right?