To me, seeing a rub means one thing. A buck has been there. That is the only thing we can know for sure. You may be able to tell differences in rubs (and the deer making them), only if 2 rubs are dramatically different (One way bigger than the other. Or one on a much larger tree than the other).

If anyone thinks they can judge the size of a buck by its rubs, they are fooling themselves. I think a general idea is the best we can hope for. Still, you really cant tell how big a buck is just by looking at a rub.