Both are 2.7in wide but s8+ is 0.8in taller. In terms of screen area s8+ is bigger by about 20%
I think it's mainly due to the 18.5:9 aspect ratio more than anything. There's just about the exact same screen area on the regular S8 as a 16:9 5.5" 7 Plus, but the Plus is easier to type on because of the width the different aspect ratio gives you.