let’s try a modern version: In reality, the sun is much larger than the moon. Also has original light, while the Moon is just the reflection. Hashem had rachmannut on the moon, and may made them appear of similar sizes. The Moon was not happy with that, did not appreciate the hesed and still complained.
Also, shows a difficulty of judging a case where you are involved. The Moon maybe had a point, but how do you express the point where you you may benefit from the solution? If the moon would would have studied mussar, it would understood that it will be punished and not rewarded. It should have offered to become smaller itself, it could have ended up being bigger as a reward.