There was some leaked Apple internal document where they said that people wanted larger phones. The post-Jobs Apple created larger phones to meet market demands.
One thing that always makes me giggle, though, is the fact that if you double tap the home button the screen slides down so you can reach the navigation bar at the top of the screen. If you have to provide a hardware solution like that, isn't it proof that the larger phone provides a worse user experience?
I'm excited about the smaller phone. My iPhone 6s is overly difficult to maneuver.