Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.
I am not surprised that diesel has a higher power density than gasoline, but have empirical reason to question the formula.
I have no reason to think there is anything at all special about my 55-Hp diesel that pushes 10-tons of fiberglass and lead around. It has routinely averaged 1 gallon per hour running at 2400 RPM of its design WOT of 3000 RPM.
Do I misread the formula?