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Mass = Stress resistance?

While continuing to browse the subject of Tyrannosaur bone density, I came across the following paragraph in an article about HP James Farlow:
Assuming that the living T. rex would have had a specific gravity (density as compared to water) between 8.5 and 1.00, the scientists arrived at an estimated mass of 6,000 kg. for the animal represented by the model. Plugging the mass estimate into a mathematical equation, the team calculated the animal's "strength indicator," a measurement of how well a skeleton handles the stress of physical activity. They found the T. rex femur claimed a strength indicator of 7.5 to 9 m2/giganewton. (A giganewton is the force needed to move 112,000 tons--roughly the weight of two steam locomotives--one meter.) By comparison, a runner like an ostrich boasts a femur with a strength indicator of 44 m2/giganewton, a muscle-bound mammalian femur like that of a white rhinoceros measures 26 m2/giganewton, and a human femur boasts a strength indicator of 15 m2/giganewton.

I think there have to be more factors than just mass in that stress equation.  Could anyone tell me what they are?  (More browsing...)