The only difference between the iPhone 11 Pro and the iPhone 11 Pro Max is the screen size and battery, and it turns out one resistor.
This is what Youtuber AoMuTech PO found when he examined the two devices and discovered by simply removing a resistor from the motherboard of the iPhone 11 Pro he is able to drive the hardware of the iPhone 11 Pro Max, allowing hardware hackers to easily upgrade an iPhone 11 Pro to a larger screen size.
This was apparently already possible with the iPhone 6s and 6s Plus, due to identical motherboards, but was not possible in later iPhones.
The discovery is obviously not useful for the run of the mill users, but I expect phone repair shops will be able to make good use of left-over components in this way.
See the video, which has a good automatic translation, setting out the upgrade procedure below: