The idea that you can determine the size of an animal from a single bone comes from Cuvier and was an early development in paleontology. I don't know how much progress there has been on the task of putting it on a firm scientific footing. http://en.wikipedia.org/wiki/Georges_Cuvier#Principle_of_the...
Lower leg bones are likely to directly relate to body weight where say teeth on the other extreme have little to so with it. Overall, it's note quite BS because it can be somewhat true.
