According to Dustin Curtis, the reason is to give users maximum usability of the phone. He starts compating the Galaxy S II with the 4.7 inch screen. Imagine yourself walking down the street, holding the phone in one hand, and looking at Google maps. That`s when you`ll feel how annoying a bigger screen would be. It`s really difficult and frustrating to reach the other side of the screen with your thumb which means you`ll have to use your other hand, while on the iPhone, you can easily and comfortably reach the end of the screen without having to flip over. See,things don`t just happen !
Since Apple treats it`s iPad as a two-handed device, it`s unlikely it will put as big a screen as the Galaxy S II `s on its iPhones.