"...divided by just the athlete's weight to the 2/3 power on one side of the equation and x divided by 2/3 power of x (the guy just standing up) on the other, something like 1350/350^.67 = x/x^.67 and solve for x; x turns out to be around 20,000 lbs."
No honest person is lesser than you, pal, none.
You "explanations" are rubbish to the core, and I'm certain you know that.
That makes you lesser than every decent person.
You have done no studies -- zero, zip, nada -- on the strengths of bones, muscles & joints in very large four-legged land-creatures, you have merely extrapolated based on what you think you know about human anatomy.
Those scientists who actually studied the question (i.e., here and here) report suffering no such fantasies as your allegations about changing gravity.
Yes, it's true, there are not a large number of land-creature fossils estimated over 20,000 lbs.
But there are some, and they include examples from the age of dinosaurs (65+ mya), Oligocene (circa 25 mya) and even into the Holocene (10,000 years ago).
Nothing in these examples suggest changing gravity.
