Once off the ground, a big jet would appear to use exactly the same resources as a private IFR flight.
Actually no. A lot of research has been done on this in the US due to the regular call by the airlines to charge user fees for GA aircraft.
The 'private IFR' flights are MOSTLY at different altitudes, speeds and distances. It is generally accepted that the average GA 'private IFR' flight uses significantly less system resources in flight than an airline flight.