Is it that older creatures were larger at that time because of oxygen?

This is due to free oxygen levels. In ancient times, 800 million years ago and around, oxygen levels were very high, so the cells were getting a lot of oxygen, and it was the giant asteroid that hit the earth that caused the high amount of oxygen in the air to decrease. It is argued that this asteroid fell 66 million years ago and destroyed 75% of the living things on earth, not just dinosaurs.

This is due to free oxygen levels. In ancient times, 800 million years ago and around, oxygen levels were very high, so the cells were getting a lot of oxygen, and it was the giant asteroid that hit the earth that caused the high amount of oxygen in the air to decrease. It is argued that this astreodine fell 66 million years ago and destroyed 75% of the living things on earth, not just dinosaurs.