The screen of the iPhone is no different and I’ve long been a preacher of the belief that it is in fact the exact correct size. Designer Dustin Curtis also sees it this way and hasposted a nice piece on his blog that lays out what he sees to be the primary reason for this:
Touching the upper right corner of the screen on the Galaxy S II using one hand, with its 4.27-inch screen, while you’re walking down the street looking at Google Maps, is extremely difficult and frustrating. I pulled out my iPhone 4 to do a quick test, and it turns out that when you hold the iPhone in your left hand and articulate your thumb, you can reach almost exactly to the other side of the screen. This means it’s easy to touch any area of the screen while holding the phone in one hand, with your thumb. It is almost impossible to do this on the Galaxy S II.
He created this graphic which illustrates the ‘average’ reach of your thumb across the screen:
(via Why the iPhone’s screen is 3.5″ and will most likely never be bigger than 4″ - The Next Web)