Thanks to Apple's lawsuit with Samsung, we're getting access to all sorts of internal Apple documents.
One of the most interesting is an internal presentation that looked at why sales growth for the iPhone was slowing.
According to the documents, the growth in the iPhone is mostly from phones with a larger than 4-inch screen, and phones that cost less than $300. Apple makes a $650 phone with a 4-inch phone.
This, plain and simple, is why Apple will release a phone with a bigger screen. It's what people want, and Apple knows it.
Chart via Statista, adapted from Apple's presentation.
Business Insider/Statista