FWIW, when AH was first released the planes with small fuel capacity, like the 109 and the La-5, had very short endurance. You needed to grab full fuel and a drop tank in the 109, even then the endurance was very short. Somewhere along the line an adjustment was made, and internal fuel capacity became less of a limiting factor. It could be that the fuel multiplier penalizes planes with large fuel tanks more than it does planes with small fuel tanks. That would explain why the Mossie has such crap range, for example.
I think F4UDOA is on to something here, but it might not be a bug. It may be the way HTC decided to handle endurance differences without making the small planes unappealling because of their limited flying time.
ra