Let me start by saying that Dunbar is an exception from the norm at 1A - IMO. I also think that any school that has a magnet program, or other vehicles to pull kids in from other school districts, has an advantage. Much like the private schools have over public schools - they have no geographical boundaries that restrict where their players come from.
All this said, I'm sure there must be some data, some real statistics that validate the premise, that on average, the more kids a non-magnet public school has, the more competitive it is with respect to a smaller non-magnet public school. If not, why even have classifications in the first place? The theory being - the larger the school, the larger the talent pool to field a team from. Of course coaching and the ability to pull talented players from other school districts will change the equation. Dunbar is one such example.