[bluGill]

I would guess that the congestion advantage goes to more smaller planes. Those from NC no longer go to NYC as a stop over when they can take a direct flight, that means the plane from NC to NYC doesn't need to be scheduled to arrive and unload in time for everyone to get the the large plane, and reload as soon as everyone from the large plane gets to the small one. Instead the only people on that plane from NC want to be in NYC and so the plane can arrive anytime so you can schedule them to arrive at different times.

That is the hub and spoke model requires the airport to have bursts of busy times as everyone tries to reach their plane at once. With smaller planes you can spread thing out more.