It all comes down to heat flux, which is measured in milliwatts per millimeter squared (mW/mm^2), and is related to the mass of the coil itself. Heat flux relates to the guage of the wire used as thicker wire(lower guage) generally has more mass and less resistance than thinner wire(higher guage). The range that Steam Engine uses for the recommended wattage sets 200mW/mm^2 as the perfect setting. If you play around with different guages and resistances from a theoretical standpoint you will find that some builds at 2ohms will use the same wattage as another build at .2ohms.
For an example I will use 2 single coil builds both using Kanthal at aiming for 8 wraps around a 2mm drilbit;
Build #1
24AWG Kanthal
8 wraps
.45ohms
Recommended wattage = about 20W
Build #2
30 AWG Kanthal
8 wraps
1.5ohms
Recommended wattage = about 8W
From this example you see that the lower guage wire has a lower resistance, which means that it requires more power to bring it up to the preferred heat flux. Resistance wires heat up by restricting the flow of electrons and causing them to essentially collide with one another turning the energy into heat. Thinner wire does a better job of restricting the flow of electrons and therefore heats up quicker at lower wattages. For this reason thicker guages are generally used for low resistance builds but if you build a high resistance coil with thin wire it can still require a relatively high wattage to run optimally.