Even though the screen only physically changed in one direction, the resolution has increased, meaning more content can be displayed. Enlarging the screen proportionally only means content will be visibly bigger, elongating the screen adds more vertically scroll-able content, meaning less scrolling, while keeping the horizontal resolution the same, thus very minimal changes have to be made for apps to work with the new iPhone. The main thing is more content is displayed. More notifications on your lock screen. More rows on the home screen. Longer notification center. Less scrolling overall, since most is scrolled through up and down, not left and right, it makes sense for a vertical increase in height. Also, making it wider will definitely hurt people with small hands. The point is that you can reach everything comfortably, for as many people as possible. Your thumb can reach further up and down more than it can left and right. Take your iPhone in your right hand and try to touch the top left corner with your thumb (top right corner for left handed people). Most people can reach that spot one handed, which is where the extra space would be. Try putting your thumb over the silent switch aligned with the front facing camera. That would be where the corner of the screen would be if width were added, a lot harder to reach.