Making the screen bigger, even in the same overall size, would make it more difficult for one-handed use because one’s thumb would have to stretch that much farther. 4” is already pushing it. The bottom chin bezel also helps one-handed use by raising the screen up (it’s difficult for the thumb to reach the bottom of the device). Basically, if one-handed operation is a major goal of this phone, which it was, they can’t change it much. They could chop off the top bezel, maybe, but it’s not really hurting anyone up there.
It’s hard to improve upon perfection.