Clarkson acts like we all didn't already know this. I tested the DNA of my dog years ago and discovered it was actually a descendant of a dinosaur. So obviously humans back then must have been much bigger, too. Everyone knows there were a few infantile dinosaurs and humans, among other species, that survived the ELE living in caves. Eventually we re-populated the Earth, but because we lived in caves for so long became much less smart, and obviously never grew to be as physically large.
The humans of millions of years ago were smart enough to see the meteor coming towards Earth before it got here, and left on their space ships, which is why there are no bones to be found anywhere. The dinosaurs that we now find the bones of were just the wild ones that weren't kept as pets, or were too large to hide with the infantile humans and dinosaurs. The rest are living in a galaxy far, far away with the humans.
The only reason they never returned is they found a better, bigger planet to inhabit.