What was the density and composition of Earth's atmosphere during the Cretaceous warmest period?
Recent analysis of the contents of air bubbles preserved in prehistoric amber reveal the oxygen composition to be 32% back then compared to just under 21% today.  These tiny bubbles have remained trapped in these amber samples for about 80 million years, which dates them well into the Cretaceous era when dinosaurs and other huge creatures roamed the earth.  

Said analysis also revealed that nitrogen made up the rest save 1% for Argon, CO2, and other trace gases. The percentage of CO2 was not specifically stated but mentioned as similar oxygen to CO2 ratio as today's atmosphere.  Whether the total air pressure air density of the atmosphere was greater back then is not mentioned, most likely because this would be difficult to ascertain from the samples.

An atmosphere 50% richer in oxygen but at the same atmospheric pressure and density at sea level as today would have allowed land animals to get by with a lower lungs to total body mass ratio and thus allowed the existence of extra massive creatures such as dinosaurs.

I assert that the asteroid that struck the Yucatan area 65 million years ago caused most of the forests and other terrestrial vegetation to ignite into flames.  The whole earth would have been on fire, at the end of which the oxygen levels were depleted to what they are today.  This is confirmed by analysis amber air bubbles from 40 million years ago, post asteroid impact, revealing an atmospheric composition and oxygen levels closer to today's.

Dinosaurs and other massive terrestrial animals could not then make a comeback since the lower oxygen content of the atmosphere would have restricted metabolism to maintain massive body sizes.