> A 12 horse lawn tractor will not pull what a 12 > horse farm tractor will.
No, of course not, for two reasons:
a) Horsepower is speed times force and determines how fast a machine can pull whatever it is that it can pull at all. "Pull" (i.e., draft) is a measure of the maximum force the machine can exert and is limited primarily by what it weighs.
b) Farm tractor horsepower is what the machine actually delivers to the wheels. Lawn tractor horsepower is the most that the manufacturer was ever able get out of the engine on a test stand in the lab.
> The larger rear tire makes alot of difference.
Tire size matters, as does tread, but weight dominates.