There is a site by David Esker named Dinosaur Theory: dinosaurtheory.com. David says that dinosaurs was so big because the early (at dinosaurs age) earth atmosphere was very dense and most mass of extinct giant reptilians and other non-avian dinosaurs was compensated by Buoyancy force from air.
the Earth's atmospheric density during the late Jurassic period can be calculated to be 670 kg/m3. This says that to produce the necessary buoyancy so that the dinosaurs could grow to their exceptional size, the density of the Earth’s air near the Earth’s surface would need to be 2/3’s of the density of water.
And he says that now at sea level "air has a low density of only 1.29 kg/m3". Thanks to Dikran Marsupial, 670 kg/m3 is very high, like density of some light wood types.
I have some doubts about so dense (thick) atmosphere and my question is: Is there any scientific evidence of Earth's atmosphere density in the era of dinosaurs?