Given that the pixel density of a mobile display is perhaps the biggest single culprit behind battery life differences, all other things being equal, that comparatively low screen resolution of the 11 works to one undeniable advantage - the phone's battery life.
Apple listed official battery life stats in the form of "up to" 15 hours of Internet browsing and 16 hours of video-binging for the XR. Not only that, but Apple was so proud of what it achieved with the iPhone XR's battery life, that it cites the "lasts up to 1.5 hours longer than iPhone 8 Plus" tidbit everywhere, as this was its previous marathon runner to aspire to.
Browsing typically means a lot of text on white background, and displaying white is the worst case scenario for OLED displays in terms of power draw, so the difference between the Pro Max and its predecessor would have probably been much smaller than the 5 hours advertised.
That is why we decided to run our own browsing test, where we loop popular websites snapshots offline and mimic scrolling behavior, to gauge the real battery life advantage of the iPhone 11 before the XR and 8 Plus. Let's not forget that all of these are with LCD displays, which don't have the same challenges as OLEDs when showing white, so going in we expected the difference between the 11 and XR to be more significant in browsing than what Apple lists for the video playback test.
As you can see, the mere 6% bump in battery capacity between the iPhone XR and iPhone 11 results in a comparable slight increase in browsing endurance. Our YouTube playback test, however, resulted in Apple's advertised hour of battery life more, and change.
In mixed usage, as in our legacy battery test, the iPhone 11 also outlasted the XR by more than an hour, hinting that there might be more to its endurance than the measly 2942mAh vs 3110mAh battery pack increase.
What's behind the better battery life of the iPhone 11 compared to the XR? Given that we have the same LCD display, and a negligible battery capacity growth, it most likely boils down to the new A13 chipset built with the second-gen 7nm process.
Anandtech recently put the two chipsets to the test, and found out that the A13 is about 25% more efficient than the A12 on average, even though it is slightly faster than its predecessor. Granted, processor efficiency weighs less in the overall power consumption of a phone, compared to the screen draw, but it's enough of a difference between the A12 and A13 to eke out gains in battery life across the board for the iPhone 11 over the XR.
The iPhone 8 Plus, on the other hand, is a good deal disadvantaged over the newer handsets - its display, albeit smaller, has a higher pixel density, its battery capacity is smaller, and the processor is a 10nm one vs the two 7nm chipsets in the XR and 11.
Oh well, the whole point of this battery life comparison is to show you what you gain if you are coming out of your two-year iPhone 8 Plus payment shackles, and jumping to the new iPhone 11 would be a significant upgrade in endurance on a charge indeed.