Humans are bipedal and so have far more stress on their two legs than even similar-sized animals on four legs. And a large difference in elevation between head and heart. Notice how huge animals like living elephants, and extinct baluchitheres ("the largest land mammal that ever lived") have their heads at about the same level as their hearts? This makes a big difference in minimum blood pressure as animals get larger. The larger the human frame gets, the further the head gets from the heart, and the higher minimum blood pressure has to go. Humans are not designed to be huge. There is no reason to think that gravity was any different in the past. Especially not the historical past, let alone pre-human eras. So, the problem that a human skeleton doesn't scale up remains the clear proof that giants of 15, 20, or 30 feet are simply not possible.
Like ancient Rome, we today are once more importing every form of exotic superstition in the hope of finding the right remedy for our sickness.
-- C. G. Jung
Richard Wilhelm: In Memoriam (1930), CW 15: pg. 60