How would the atmosphere of earth differ if the earth was much larger

If the earth just doubles its size in an instant, the oxygen present in the atmosphere could be much larger. Due to these, we could theorize that insects could grow much larger because the size of their body is heavily dependent on the maximum amount of oxygen present in the atmosphere. Air pressure also increases thus those aerial enthusiasts would enjoy their professions.

It would most likely be harder to breathe and we would probably not get enough sun