I don’t think there is a scientific consensus. However, I think past levels of CO2 and global temperature are a more likely cause of gigantism during the middle and late Mesozoic than differences in O2. Levels of CO2 and global temperature were generally high during the Jurassic and Cretaceous. From here:
Increased global temperatures and higher CO2 levels would likely cause much higher primary production by green plants. Because of this, herbivores would find it easier to obtain enough food within a particular area. Because they could find more food with less effort, they would be able to grow larger. With larger herbivores, carnivores would also be able to get larger.
The KT mass extinction wiped out all almost all land animals larger than 5 kg. It took millions of years for mammals to become much larger. Climates during the Cenozoic have generally been cooler than in the Mesozoic, so land mammals have never reached the average size the dinosaurs did.