The older engines had heavier crank assemblies, was harder to pull the engine down under load, hence better sustained torque under load.
Heavy flywheels help only for a very short period, though long enough that if you know what you are doing and "listen" to what is happening you can back off.
A sustained load will eventually load an engine down.
To the O/P's question;
It is MOSTLY about "Pulling" force and the most limiting factor to that is usually tractor WEIGHT, more accurately load on the driving tires.
Tire ballast is a very effective and low cost way of increasing this, adding as much as 25% (maybe more) to the weight and it is LOW, so also helps stability.
Just about any tractor you can buy will be geared low enough in it's lowest gears to spin it's tires at very low engine speed (some even on black top), so "FORCE" is available, but it isn't effective until it is coupled to the ground.
Why "Horse Power" as a unit of measure ?
Well, for tractors it might have made marketing sense toward the end of the horse drawn era - today ? I suppose it depends on whether you relate to automobile horse power.
I am told (though it was "before my time") that tractors used to be rated by their plow capacity and that was a more meaningful measure of farm work when figuring if they could replace horses. There were "One bottom" and "Two bottom" tractors, meaning they could pull one or two shares (through some uncalibrated soil type to some unspecified depth and width).
In REAL HORSE terms; I am told by a friend that the Amish near her have a nine horse hitch. I have never seen it, since I have never been there when they are plowing.
The THOUGHT of nine draft horses plowing is kinda awesome (to ME) and a reminder that many agricultural tools have evolved from horse drawn versions.
e.g. the design of small plows has evolved from plows used at walking speeds.
Not to get too carried away with this, but nine Belgians have traction largely because of the WEIGHT they put down and the very small area that they put it down on, i.e. PRESSURE, they DIG IN !