The smaller diameter of the wheels helps in extending speed in lower gears. To use an example. This is what a friend found back a couple years ago. He used to race his SVT at the track.
With the stock size 17's he would have to shift out of 4th towards the end of the track because he was no longer gaining any more speed out of the run for a short period before the end of the 1/4mile.
By switching to 15's drag wheels he was able to stay in 4th to the end without loosing any power.
It has been awhile and I do not know the specifics off the top of my head but it all has to do with the diameter in which wheel has to cover to make a complete rotation.
I am sure someone will post a better example/definition.
2002.5 Silver Euro ZX5