You may have heard rumors that the next iPhone will have a larger screen, however, the latest comment from Apple’s chief executive Tim Cook hinted that the next-generation iPhone will not be any bigger. He believes the iPhone 5 has the “right” screen size.
“It [the iPhone5] also provides a larger screen size without sacrificing the one-handed ease of use. We put a lot of thinking into screen size and we believe we have picked the right one.” – Tim Cook (from abcnews.go.com)
One platform for developers means better control of the apps or programs that run on the device. This leads to better performance compared to Windows or Android where it has to accommodate a number of variables (screen size, CPU, GPU, etc.).
Jobs may have used the theory of one hand control for the reason of the 3.5 inch screen (and Cook continues to use it for the iPhone5 4” screen). Behind the curtain, what it really means is Apple wants a single platform for developers to work on to maintain maximum optimization. You can read the full article here.