<font color="red"> My Kubota is 37hp. </font>
Gatorboy's answer is correct, but I'm confused about your tractor. Is the engine HP rated at 37 or is it the PTO HP that is rated at 37? If the engine HP is 37 then you probably have about 32 at the PTO and you would be in terrible shape with that bush hog. If the PTO HP is 37, you'd still be in bad shape, likely could turn it in grass but I wouldn't want to try taking over much brush or anything heavy.
EDIT and some of what Bill asked just before me also puzzles me. The gearbox is often rated above the tractor's required HP so what is it that you are referring to?
Also, your question about the difference between light and medium duty . . . usually it is more than shear weight often it is the weight of the flywheel, the number and thickness of the blades, the gearbox assembly, etc.
It may end up that your tractor can easily handle this . . . or not.