how much more "capability" can you add in such a little space? That's enough for what, maybe 5% more battery time?
Not even. The last number I saw was about 2% for an iPhone. Compared to something like 20% through making the entire phone either 1mm or 2mm thicker.
Apple being Apple, it wouldn't surprise me if they remove the headphone jack, increase the thickness of the phone a smidgen, and claim the "innovative" decision to remove the jack led to a 10% increase in battery size.