Of course generators take less fuel when powering smaller chargers, which raises a question:
For any given generator - what is the ideal load to place on it (i.e. what size RV battery charger) for maximum fuel usage efficiency?
This is exactly the situation with an autombile (not considering air friction) having a sweet spot speed to cruise at so as to optimize fuel mileage.
As an example, our little Honda is only partially loaded by our RV's Parallax converter when charging our batteries. However the Honda is slightly quieter, and it runs a lot longer, from the partial loading that the converter places on it. Maybe the maximum charger for the shortest generator run time is not quite the proper setup for ultimate charging efficiency with respect to the amount of fuel used for the given task.