I know why he used it...
Because it acts like intercooler pipe size does. A larger diameter "throttle body in this instance" would decrease the amount of boost generated (or PSI drop) and also increase the boost lag.
This is another example of when larger isn't always better.
Also this only applies to forced induction engines. A N/A engine pulls air in and then a larger opening would allow for more air flow. Velocity & pressure drop are not a factor.
