I believe it is normal in all aircraft. This is why the higher you go, the less fuel the aircraft uses (saves money for the Airline company). All airliners have an optimum and maximum altitude at a given weight (seen in the VNAV Cruise page in the FMC on the 757 & 767), so I always try to fly near the optimum altitude. Lou can explain it better than I can, so I will leave it to him. I understand how it works, but I'm not very good at explaining how it works. :-/

Mark