port calculations

why is it that a 6 inch round port tuned to 35 Hz needs to be 13 inches long in a 4.5 cu ft box but a 2 inch round port only needs to be a little over an inch long?? what is up with that? how does that work?? it seems logical that the 6 inch port would have to be shorter because it can flow more air. what is a good port size to use? i have many different types of pvc and i am going to experiment with the box in my truck and try to port it. im using a hollywood excursion 15 a spl 820 amp and a 4.5 cu ft box. oh and i dont know if this matters a alpine cda 7892 h/u. the BLACK/ green flip down unit. thanks for the help.

           wes