I think gigantism was a common trait across the world in different sub sets prior to the Younger Dryas cataclysm. The inverse would be the pygmies of Africa, and Hobbits of Indonesia. Megafaunal species around the world died off after the same time period, with Africa being the least effected.
I think the main force behind this scaling down may be a worldwide increase in gravitational pressure. The Earth's rotation may have been dramatically altered in the Impact/Micro-Nova event, which may have led to the gravitational changes. This is why the giant humans that survived passed on the recessive trait, with most modern examples having very poor health and eventual heart failure.
It's been calculated that pterodactyls could not have gotten off the ground in today's gravity, based on their weight and wingspan, and Brontosaur types would not have been able to pump blood up to their heads, the pressure needed would explode their arteries. Geological layers are pretty well studied, you can go to the American southwest and know whether the bones your digging out are from dinosaurs buried millions of years ago, or giants buried 20,000 years ago based on layering.
Tartaria is interesting, I haven't read Fomenko's theories but I'm open to the idea that vast swaths of recent and ancient history have been completely erased from our education.