I've never understood the American model of paying more,as the plane grows bigger. Did medium haul in a 150 seater for many years. On a good day, I moved 550 pax. On a really bad day, 300. Now - why should the longhaul guy who on a good day, moved 300...get paid more??
Liability. Why should a guy driving a $300,000 Ferrari pay more for insurance then a guy in a $14,000 Hyundai? Why would a family in a 12,000 sq ft house pay more insurance then a family in a 1,200 sq ft house? If a pilot is responsible for moving 300 people he carries a LOT more liability than a pilot moving 150.