Your tractor can DRAG far more than it can lift. Gravity and the earth are your friends. If you try to cleanly lift a 800# piece of concrete, you must be able to LIFT the entire 800#. Hhowever, if you DRAG the same thing, you can get by with lifting one side or probably only 400# of weight. Angles and heights will change all this, but the basic thought is the same. Let 400# drag on the ground and only have to LIFT the other 400# Get one side 3-5" off the ground and you are good to go. If the bottom is very rough, flip it over to get a smoother skidding surface.
Think of a block and tackle combo. Once you double the line, you cut the lifting requiremnet in half. That is how a 125# weakling can lift 300# with a block and tackle with multiple lines.