Phones with larger screen's generally have better battery life than phones with smaller screens. Look at the iphone 6 vs 6+ for instance. Increasing the screen size leads to disproportionately more room for battery since most of the other components dont get bigger as the screen size does.

I agree that we are at a time when battery life should be prioritized, but I would prefer that to come from thickness. Increasing the thickness by just 25% or so would allow a much larger battery.