But there is a differense.. even if the weights wouldn't damage the sensetive weightbalance in a horses leg (and let's pretend horses instictively know how to work with these devices without straining themselves in a bad way), you still have the added 'weight' from the gravity (centrifugal force in swedish) when the leg moves in these speeds.. plus it is a harsh, metal chain that gets slammed on sensetive skin over and over again. You try running as fast as you can for a mile with loose, heavy chains on bare feet and legs. If you still say it doesn't hurt I just might reconcider the horse issue..
And comparing the chains weight with the horses weight against humans weights etc.. it just doesn't work that way. For one.. the weight on us are divided very differently. For two, the horses body are a work of balance in a much different way than ours.. and.. size really doesn't matter that much in this case.