I suspect that they invite focus groups in to hold and examine mockups or prototypes of different form factors that Apple has built in-house, and use that information to decide what their next model will look like. They probably also examine the trends that are succeeding with their competitors, such as Samsung. Deciding to arbitrarily include a bigger battery without knowing whether that would actually sell more phones would be foolish.
Apple has historically abandoned focus groups. From Jobs in 1998:
"It’s really hard to design products by focus groups. A lot of times, people don’t know what they want until you show it to them.” (Business Week, May 25, 1998)