As I understand it, the proof is how tightly they pack the pixels. If you need a stronger backlight to illuminate the screen, and more power to push all the pix

As I understand it, the proof is how tightly they pack the pixels. If you need a stronger backlight to illuminate the screen, and more power to push all the pixels, it stands to reason that a more detailed screen e.g. 1080p instead of 720p, in the same form factor, is going to use considerably more battery. It's part of the reason why the iPhone 6 does better in battery life tests without gimmicks to extend the battery life as the phone hits Low Battery warnings, the screen's resolution is just over 720p instead of the Nexus 5's comparable 1080p screen. This difference is highlighted by how the Nexus 5 had a 2300 mAh battery while the iPhone 6 has merely 1810 mAh, 20% smaller. In battery tests for the 1080p Nexus 5, under L, it lasts roughly 8 hours. The iPhone 6 gets about the same playing movies, maybe two hours longer otherwise. I'd argue the screen plays a huge role in this.