This question seems to miss the understanding that horsepower is a measure of work done- that is, some amount of mass multiplied by some amount of distance all divided by some amount of time. (Horsepower a a unit is kinda weird like most US customary units, so you would have to pay around with the numbers a bunch to get mass, distance, and time out of the force, rotational speed, and constant that make up this particular unit)

In an application where your work gets done at a slow speed and doesn't need to change that speed significantly or quickly like towing or farming, a better unit of measurement to use would be in units of force. How much force can this machine apply to this load. For things that rotate, the force is torque.

Horsepower is calculated from torque and rpm. The equation is (torque × rpm)/5252. If all the torque needed is generated at a very low rpm, as is the case with large diesel engines, the horsepower is going to come out small.