A reasonable point Racingnovice but of course there are many more factors at work which will decide who finishes in front. I guarantee you I, at 70kg, can easily account for some 100kg blokes around no matter what weight is being hauled.

Of course I would reach my optimum weight carrying level and then I would tire considerably. I think that is the point. Some horses can carry weight, some can't. It is no secret that better class horses can and still beat lower class animals. I think this is why horses going up in weight win more races, they are better horses.