I draw this line somewhat arbitrarily at around 16,000 lbs or 35 feet. Diesel becomes the better choice in direct proportion to the amount of weight being propelled. Horse power and torque are two different measures of power. Torque is a measure of the kinetic energy that builds up in a rotating engine. The higher the torque, the more power it takes to slow the engine down or, in other words, it takes more power to make it work harder or, the engine will carry a heavier load with less strain.
Diesel engines develop more torque for several reasons.
But they also have compression ratios three times that of a gas engine, which also develops more torque. Thus the diesel's great advantage is carrying more load with less strain on the engine due to higher torque generated. When dealing with lighter loads, that advantage disappears. There is also an issue of kinetic energy, which is energy that builds up in rotating parts such as flywheels, which helps sustain the load. Another advantage is that the diesel will develop that power with significantly less fuel.