There are two explanations to Moshe's reminder that we were not chosen for our special role among the nations because of our great numbers. On the contrary, we are very small in numbers.
The של״ה הקודש explains that our low numbers emphasizes the importance of humility. In other words, Hashem does not love us because of our numbers, but because of our humility.
Leaders of the Gentiles like Nimrod, Pharoah, and Nebuchadnezzar, displayed great arrogance.
Our leaders displayed humility. Avraham Avinu said that he was dust and ashes. Moshe and Aharon said, אנחנו מה, what are we? And King David said that he was a worm and not a man.
The second explanation of our small numbers is to teach us that we must remain separate from the other nations. When we maintain our unique character and way of life, we will experience greater blessings than the other nations.