See: http://www.displaymate.com/Smartphone_ShootOut_3.htm. At the same brightness level (Anandtech uses 150 or 200 nits I think), the FHD displays use only 15% more power than the iPhone display despite almost 3x the pixels. And total display power is probably about half the power budget.
Read how that test is run. It's showing a static image. There's nothing changing, there is no rendering in that test at all. So of course resolution didn't have an impact, the test is built to test purely the display's power draw and does excactly that.
