Rene Ritchie at iMore:
One of the most common misconceptions when it comes to the iPhone is that Apple could simply make it a few millimeters thicker to improve its battery life.
It then follows, under that train of thought, that Apple is deliberately sacrificing battery life for cosmetics. That the company is putting an obsession for thinness over usability. In fact, it's the exact opposite of that.
Having used a number of battery cases for iPhone 6, I would not want it to be anywhere near that thick for the sake of its battery — I much prefer adding that type of case if I know I'm going to need it. As Rene argues on Twitter:
There’s no thick iPhone, so mophie. There’s no ruggedized iPhone, so Otterbox/Lifeproof. Apple provides the core. Add what you want.— Rene Ritchie (@reneritchie) February 24, 2015