"The size was chosen specifically to work well with the human thumb and be ergonomic" when apple was making the phones smaller a few years ago. Apple apparently isn't very good at consistency.
They were faced with a problem though, consumers appear to be flocking to larger screens in the 4.5" to 6" range and it was either be considered small by new buyers and rely solely on the current customer base or try and join the large screen crowd and draw back some of those that want large screens and some of those that left due to them not having the larger screen. Personally I just don't like the smaller screens, I have a 3.5" android and it's hard for me to read anything on and an eye strain to boot, I'd much prefer a larger sized one on my next go around. Apple having the supposed 4.7" model will mean I have to choose between it and android instead of just android. (I also have a 4" Windows Phone but it's not got the apps I need.)
Yeah, but it's fun to make fun of them when they made such a big deal about how much better the not-large screen was a few years back. Honestly, I don't see much of a future for the I-phone. The whole "open source" system with android has created way too much competition-based innovation for apple to even want to keep up. It also has produced a crap-ton of 7 inch phones, slow touchwhiz, etc, but that's the price of market selection. Gotta take risks to make strides.