for technical folks here is a good article on the math that goes into simply calculating the guide veins on the inside of an airbox. If you look at the m3 airbox inside, there are nicely curved shaped pieces of plastic rather than simply a flat surface. This is just one of the many components that takes a bunch of math to minimize the pressure drop between incoming air and air after going through the filter.
You can be sure that M knew exactly the diameter of piping needing and wanted to maximize engine efficiency and power.
I can promise you macht schnell or AFE etc does not use complex computer modeling to adjust size and design for all of the factors that go into an airbox design.