Ultralights:
Would that not just reflect the size of the aircraft being used?
ATC control aircraft, not passenger numbers.
You should be directing that criticism, if it is one, at the airline companies if anyone.
The blip on the radar makes exactly the same amount of work to ATC if it is an A340 or a Jabiru.
(In fact the Jabiru takes longer to process and probably more work if you wanted to really argue the point)
How would you calculated in an MD11 Freighter using your logic?
Or GA Aircraft that are not cpatured by passenger number statistics?
Perhaps you have some fuzzy logic in the ATC efficiency equation that I have missed.