I'm no engineer, but are there any limitations to how a battery must be shaped? Because looking at volume alone, they'd be able to double the battery size without adding more than very little to the thickness. If the battery must be shaped like a brick, they could simply add a second battery and place in on top of everything else, making the phone a bit thicker.
Does anyone know if there is a practical/technical/economical reason for them not to offer twice the battery capacity?
Make the 5 as thick as the 4, and use all that extra volume for battery. Volume wise, adding 1,7 mm to the phone's thickness gives space for a battery 1,2 times the size of the current battery.
I'd definitely prefer that!