Two major factors contributing to the difference in delivered temperature. Wood density and Heat Exchanger Geometry (and materials).
I have experimented with a wide variety of woods (including Myrtlewood, Ebony, Pink Ivory, several Rosewoods, Bocote, Tea Tree, Ash, Walnut, Maple, Western Maple, Rose Sheoak, Red Oak etc.). Woods that are 'too' dense do not provide adequate insulation. This has been discussed in detail in the PD experience thread. Wood density has a much greater influence than diameter (and height has almost no effect). We use Kiln Dried American Cherry for good reasons.
The Heat Exchanger geometry (and materials) are the other obvious contributing factors. Tighter tolerances, massively increased surface area, dark fins, restricted air intake and heavy wall tubing all contribute to the increased heat exchange rate.