The size of the plane used is more dependent on the amount of demand in a market than the length of the flight, I think. After all, if an airline can fill a 200 seat airplane on a 10 mile flight (an exaggeration, I know, but you get the idea), then why would they schedule only a 34 seat turboprop. In the cases of city pairs where demand does not warrant the larger aircraft, I can think of one reason that has not been listed as to why an airline would use a larger plane than neccessary... American used to fly DC-10's and 767's between LAX and SAN. They would fly the latest flights LAX-SAN, overnight in SAN to be washed, and fly back to LAX in the morning. Airlines, (American, at least), only wash their planes in select cities, so they need to get the planes to those cities. The DC-10's, for example, didn't serve LAX-SAN until the JFK-SAN route was downgraded from a DC-10 to a 767. Since the DC-10 wasn't coming from JFK anymore, they had to get it to SAN from elsewhere. Incidentally, I have flown on a DC-10 from SAN-LAX that had only 2 PAX - my sister and me... coach and business were totally empty!