Just doing some quick math, I took the approx weights and engine HP for my tractors, and they turn out like this:
1966 IH cub 14hp / 14xx lbs
NH 1920 33hp/ 3500 lbs
ford 8n 27hp / 2800 lbs
YM 1700 ? 20hp/ 1800 lbs
That is floating about 10 pounds per hp. I've noticed the japanese tractors tend to be a bit 'thinner' and the chinese /korean/indian tend to be 'thicker'.. guess it is due to the metalurgy of the castings.. needing bigger /thicker heavier castings to achieve the strength , etc.
Soundguy