300 sqs makes it more of a "Pilot's Event" and it deters "engine cheating" because the extra drag penalizes attempts to go faster than the guys who fly legit.
An arbitrary figure..? Maybe they tried a few sizes out before voting...?
I'm sure it's still plenty "sporty" to fly and to watch.
It's just as important to give the "also rans" a feeling like they have a shot, or else you find yourself racing all by yourself.