I would swagger most of the HP use is the soft ground vs the actual weight of the implements. Soft soggy ground will require 2-4 times the HP needed to move a give load vs a hard surface like asphalt or concrete. There is even a difference in HP needed between these to road surfaces, as there is between green or fully curred concrete. With Green needing more HP to move a given load.
I can move a 100,000 lbs of weight with a 10HP motor at 1mph, on harde ground, with proper gearing etc. Soft spongy material will take way more than that!
Or in the case of slow moving items, HP is not as I know it to be the true mover, torque and the multiplication of it thru gearing, or a hydraulic setup so the torque as to a degree infinitely multiply able.
Marty