Where are you seeing this? On the Apple site they list the 12 pro as being 146.7mm x 71.5mm whereas the 11 Pro was 144mm x 71.4 mm. That's a larger surface area than the 11 Pro, not smaller.
I was comparing the 12 and 12 Pro to the base 11 and XR, not the 11 Pro. Base 11 and XR had the same 6.1" screen size as the 12 lineup, the 11 Pro has a smaller screen (5.85", identical to the X/XS and not present in the 12 lineup anymore).
