The Chasam Sofer offers a fascinating explanation. He asserts that the moon was created the same size as it is now, and with the ability to shine the same amount of light as it currently does, for this was what the world needed. The only difference between the moon before it complained and after that, is that Hashem took away the moon's independent ability to shine. The moon used to have its own light, however, after its complaint it was only able to reflect the sun's light. Even though its light was not initially the same as the light of the sun, Hashem still referred to both of them equally as שני הַמְּאוֹרֹת הַגְּדֹלִים. That is because size and ability are completely not relevant to Hashem. A creation is only גָּדוֹל great if it does the will of Hashem. Therefore, both the sun and moon were equally great, for they were both fulfilling the purpose for which they were created!
The moon actually understood this and knew that its light is less than that of the sun and that its purpose is different to that of the sun. The moon did not object to this reality. It only objected to being referred to on equal footing to the sun; שתי מְאוֹרוֹת הַגְּדֹלִים, when in reality it knew itself to be smaller. But Hashem answered that its smaller size and lesser ability to shine does not in any way mitigate its greatness! And if the Torah will refer to it as the מָאוֹר הַקָּטֹן due to its smaller size and lesser light, then it will cause people to stumble and think that their abilities actually have an impact on their greatness.
This fact is far removed from reality. The only true measure of greatness is whether a person is fulfilling his purpose in this world. Therefore, Hashem ultimately took away the moon's ability to give light, in order to not allow man to stumble and think that size, quantity, or accomplishments are the factors that measure greatness!
The Chasam Sofer explains that a little creature such as an ant, and a large animal such as an elephant, have equal value in Hashem's eyes. They are both simply fulfilling their roles in creation. אַל יִתְהַלֵּל הַגִּבּוֹר בִּגְבוּרָתוֹ אַל יִתְהַלֵּל עָשִׁיר בְּעָשְׁרוֹ (ירמיהו ט כב) a wealthy man should not pride himself with his wealth or a strong man with his strength, כִּי אִם בְּזֹאת יִתְהַלֵּל הַמִּתְהַלֵּל הַשְׂכֵּל וְיָדֹעַ אוֹתִי (ט כג) one should only pride himself if he uses his gifts as a means to recognize the greatness of Hashem. For that is the only definition of greatness. Overcoming one's evil inclination and fulfilling Hashem's will in the place and with the circumstances He has granted us.
An Ideal World
When the world will reach its ultimate state with the coming of Moshiach, this concept will be clear. It will be obvious to all that the rich, the smart and the strong are not praiseworthy for the gifts they have been granted. Rather the fulfillment of Hashem's will, regardless of the circumstances, is what defines greatness. Therefore, Hashem will be able to restore the moon to its original light, for man will not stumble and think that equal ability means equal greatness. Thus, אוֹר הַלְּבָנָה כְּאוֹר הַחַמָּה the moon will be restored to its initial light at a time when the world will see clearly that the sun and the moon are indeed equal, for they are fulfilling the roles Hashem gave them.
(חתם סופר פר' בראשית ד''ה את שני המאורות הגדולים)
This is a crucial concept to internalize in order to overcome our basic animal instincts. For only if we have a crystal-clear awareness of our constant ability and obligation to be our best selves in any situation, can we consistently strengthen our neshamos over our guf. We are only obliged to follow the course upon which Hashem leads us. By faithfully following this path, through consistently acting according to our higher values as opposed to our natural desires, we will bring closer the time when אוֹר הַלְּבָנָה יִהְיֶה כְּאוֹר הַחַמָּה. Amen.
גוט שבת