If we are talking about screen size, Yes, iPhone 12 Pro Max at 6.7-inches has a bigger screen than iPhone 8 Plus at 5.5-inches. However, in dimension, they are very close with iPhone 8 Plus dimension at 158.4 x 78.1 x 7.5 mm and iPhone 12 Pro Max at 160.8 x 78.1 x 7.4 mm.