Different People Have Different Hands

Apple notes that the top left and bottom right areas of the new iPhone's screen can easily be touched with your thumb, without contorting your hand.

I've seen a number of iPhone reviews talk about this. Some noted that the screen was uncomfortably big, others reported that the phone had just the right screen size, and even others preferred larger Android phones.

It's a bit of a strange claim either way, because different people have vastly different hands. Apple probably strikes a good balance, but it's not surprising that, for many, the iPhone is either too big or too small. There is no one perfect screen size that fits everybody's hands.