What is happening is you are loosing that energy you put in to heat. The 3900 kv motor is used for a couple of things:1. A heli motor where it is geared way down before going to the rotor. 2. A motor for use in a ducted fan, where the impeller diameter is very small (maybe 55mm ~ 2 inches) or 3. a plane like you have (a funjet) but using a 2S battery instead of a 3S. The higher kv motor in the same sized motor will generally handle more current, because there are fewer turns of wire, so the wire in the same stator can be of larger guage and handle the increased current.

You can think of the number of turns kind of like the gear ratio in a transmission. The more turns, the higher the gear ratio, and the lower the output speed of the motor given the same voltage. Note: this doesn't perfectly equate across product lines, but for the same design motor with the same number of poles, it is generally true.

Confused yet? Finding that "perfect" motor is impossible, but I think with a little tinkering you can find one that will work well for you.
Curtis
P.S. On that first chart you posted, did you see what happened to the current? Try putting your 3900 motor back in there with the 4.75x4.75, but change the number of cells in your battery to 2 (like in bullet #3 above) and see what you get.