In my original question I meant that, if you look at the sales literature or data sheet, there are many numbers to describe engine, frame, hydraulics, PTO, transmission, and on and on, arranged in a neat table. And if you look across models, every single number is exactly the same, except for the horsepower. They report weight to the pound, for example. If the higher horsepower tractor had an added turbocharger, there would have been more pounds. Unless of course the numbers are wrong, in which case there's no way to compare specs.
The idea that a higher horsepower model would consume more fuel is something I didn't think about, and it's an excellent point! Except that if the 40, 50, and 60 horsepower models all consumed the same fuel when actually delivering 25 horsepower, then it doesn't matter. I don't know if this is the case.