Believe it or not, it's proportional to the fourth power (!) of the axle load: https://en.wikipedia.org/wiki/Fourth_power_law
Does a sharp pointy thing poke a hole in a surface more easily or less easily than a big flat thing?
The "list of academic and government citations" is all just finger-in-the-air unscientific "we think it works like this because it fits our primary school child view of the world".