Torque and Horsepower

I understand horsepower as an abstraction — as something James Watt guesstimated two hundred years ago in order to sell steam engines to mine owners — but what it means in terms of comparing a 90 HP outboard motor to a 90 HP engine in a tractor escapes me. The two engines don’t seem the same to me: the outboard engine is smaller and, it seems to me, is not capable of the same amount of work. I need to ask someone like Gerard Olinger about this. Until then, I have done several searches in hopes of getting a decent answer to my question. This [article][] offers me the most insight.