Why is it that the more soda that's in a closed container, the less flat it becomes?

Question

For instance, if only some pop is in a 2 litre container of soda, it becomes flat quickly, but if it's say nearly full, it hardly gets flat. Why?

Answer 1

The carbon dioxide (bubbles) in the soda want to reach a balance between what is in the soda and what is in the air space above the soda.

The more space that is empty the more carbon dioxide that it will take to reach that balance.

Answer 2

because of the amount of air that gets into the container...the less air the less flat....

Answer 3

The more a soda is opened and used the more carbon dioxide is let out.