Horsepower is derrived from a mathematical function where torque is multiplied by RPM and divided by some constant. It's why a Harley Davidson can make a ton of torque down low, but have much lower HP numbers than you would expect - they just don't rev very high. A tractor is going to be similar. You want a boat load of low down torque off idle so you can plough through dirt and mud, but you don't need it to go very fast, so there's no point in making it rev as high as a car. If those engines could spin as fast as car engines, they'd make substantially more power, but they don't spin very fast, so the HP number is low