Celestia, Cadance and Luna compete to see who can get massive enough to step off the planet. The three compete using methods to not only try to make themselves grow, but to thwart the other two. The atmosphere fills with countering magic fields making it difficult to progress. Whoever would reaches the edge of the stratosphere though, will have access to a zone that is free of those suppressing effects.
 
Luna reaches that edge first. Now free of the other two’s thwarting effects, she grows at an explosive rate gaining both size and power. She puts a barrier on the planet to prevent their growth.
 
Luna wins.