Unported head quickness?

hi, how do you figure 1 7/8 are too big? I ran 1 3/4 , switched to 1 7/8 , picked up solid .2 and 2 mph faster over the smaller headers. the 1 7/8 do require a longer collector. later on , i went from 3 " dia collector to 3 1/2 " dia collector, gained . 05 and almost 1 mph. just food forthought