Real world usage will always differ from manufacturer claims, regardless of what industry you’re looking at. However, Apple’s claims have always been a pretty reasonable estimate of real world usage.
Therefore, the battery life on the XR should definitely be substantially better than the XS & XS Max. Again, this can only be verified after the phone is launched.
The fact is that XR is already cannibalizing XS. Not the Max. The XS isn’t selling very well at all. Loads of people are waiting for the XR. Take China for example, the XR comes with dual physical SIM compared to the XS, it’ll definitely cannibalize the sale of XS based solely on this point.
https://9to5mac.com/2018/09/19/iphone-xr-shipments/amp/
Let me reiterate, improve in battery life is NOT due to screen resolution. It’s due to the the display panel. LCD vs OLED. LCD consumes less power as compared to OLED.