You've got to remember that tractors and heavy equipment come in all sizes, not just small (by todays standards) old farm tractors. I think they might have been used on big equipment, which require a couple tons or more of weight. Think gravel pits and monster front loaders. Those things take real weight!
We always used weights on my Dads old Farmalls, and they always had the rounded corners like that, but they were never that big. I have similar chunks of iron/steel that I use to hold down stuff I don't want blown away in a tornado or hurricane, but they have sharp square edges. I have no idea what they were for, but they sure come in handy at times. I kept them in the back of my truck to keep stuff from blowing out. They're toe and finger crushers though, so you always have to be careful moving one! And there's NO picking them up....you scoot or roll them.
