Right on this one also. But there is a shortage of engineers too as well as pilots and secondhand aircraft. Non knowing the B757, why would it be any more labour intensive than a B737?
1) More seats, more pax = more cabin defects.
2) More wheels and brakes = more wear.
3) More doors with more complicated escape slides = more time to change a slide or lube a door.
4) You even need a set of steps just to get in the E/E bay or service the Hyd which all take time.
5) different inspections criteria ie. Eng MCD's are more frequent than the 737.
I could go on but I think you get the picture.