The reason that barndoor ailerons produce more adverse yaw is simply that in producing the differential lift required to roll the airplane, less of the wing is involved. Getting the increased lift from a smaller portion of the wing requires a proportionally higher lift coefficient from that part of the wing than would be required if more of the wing was working in concert with the aileron. Since induced drag is proportional to the square of the lift coefficient, the yawing force produced by the lowered aileron is roughly inversely proportional to the amount of wing involved. If you reduce an aileron's span by, say, 50%, you will need roughly double the increment of lift coefficient to get the same rolling moment, and doubling the lift coefficient will produce four times the induced drag increment per unit of area - the net result is considerably increased yaw.

Full scale wind tunnel tests conducted in the early 30s indicated that ailerons spanning roughly 60% of the wing produced the necessary lateral control with minimal adverse yaw, but this doesn't leave very much room for wing flaps, so a compromise is made, with the ailerons shortened. Airplanes without flaps perform best with ailerons covering most the the wing span - a good examples are the unlimited aerobatic full scale birds such as the Extras and Edges. Full span, relatively wide chord ailerons are also very effective at counteracting engine torque, enabling flying rather deeply into the stall with good aileron control, simply by deflecting propwash against the roll, as evidenced vertically nose-up hovering with no rolling.
Barn door ailerons produce much more adverse yaw, and are almosts useless for hovering without torque rolling.