the atmosphere has changed over time, considerably. For example, samples drawn from millions of years old amber show well known differences.
Air Bubbles
But there was no sign of gigantism in the past, particularly related to today. While there were big ass dinosaurs, there were also small ones right beside them. Many critters today are among the largest ever. Some of the rain forest trees and vines, like the Strangler Fig, would definitely be classed as giants.
Aslan is not a Tame Lion