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His answer, via Seeking Alpha:
My view continues to be that iPhone 5 has the absolute best display in the industry. And we always strive to create the very best display for our customers. And some customers value large screen size, others value also other factors such as resolution, color quality, white balance, brightness, reflectivity, screen longevity, power consumption, portability, compatibility with apps and many things.
Our competitors had made some significant trade-offs in many of these areas in order to ship a larger display, we would not ship a larger display iPhone while these trade-offs exist.
It's an interesting comment because our gadget guy Steve Kovach says the screen on Samsung's 5-inch Galaxy S4 is better than the iPhone 5. He also thinks the HTC One's 4.7-inch screen is better than the iPhone 5.
Perhaps the trade off Cook didn't want to make was producing a slightly thicker phone. Both of those phones are thicker than the iPhone 5.
For many consumers, a thicker iPhone would be worth it to get a bigger screen.
The good news is that Cook didn't shut out the possibility of a bigger iPhone coming down the road.