וַיֹּ֖אמֶר אֶל־עַמּ֑וֹ הִנֵּ֗ה עַ֚ם בְּנֵ֣י יִשְׂרָאֵ֔ל רַ֥ב וְעָצ֖וּם מִמֶּֽנּוּ
Netziv points out that at this point in time, what Pharoah was saying was categorically false
אע״ג שהיו אז עדיין מעט אנשי חיל מישראל נגד מצרים, ולא היה לחוש לעת עתה שיתגברו על מצרים
but Pharoah was anticipating what would happen given the growth rate of the population. Ohr haChaim offers a different answer:
ואומרו רב ועצום פירוש כי באמצעות היותם עם אחד ומיוחד הגם שיהיו מעטים בערך שאר האומות יחשבו לרבים ועצומים מהם לצד שהם כל אחד נותן נפשו על אחיו, וזה ידוע ליודעי ערך מלחמה
Strength is not just a matter of numbers. A large force that lacks cohesion can be overcome by a smaller force that is unified. Bn"Y may have been small in number, but they had an unmatched cohesiveness as a people, and were therefore considered רַ֥ב וְעָצ֖וּם. Every individual was willing to give even his life for the sake of his friend.
True then, still true today.
With this Ohr haChaim as background, we can better appreciate what happens later in the parsha where we read how Moshe came across an Egyptian who was striking a fellow Jew:
וַיִּ֤פֶן כֹּה֙ וָכֹ֔ה וַיַּ֖רְא כִּ֣י אֵ֣ין אִ֑ישׁ וַיַּךְ֙ אֶת־הַמִּצְרִ֔י וַֽיִּטְמְנֵ֖הוּ בַּחֽוֹל
The Binah laItim explains that Moshe saw that there were other Jews who were standing there who witnessed this horrible attack, and yet וַיַּ֖רְא כִּ֣י אֵ֣ין אִ֑ישׁ none of them were willing to stand up for the victim.
Moshe was willing to give these onlookers the benefit of the doubt. Bn"Y are by nature rachmanin. For better or worse, we avoid getting involved in street brawls or violence. This was undoubtedly all the more true considering that the onlookers in this case were slaves who were subjugated by the Egyptians, and so psychologically they would have been all the more hesitant to take a stand against them. Since no one else did anything, Moshe took it upon himself to be the אִ֑ישׁ where no one else could. As Chazal tell us, במקום שאין אנשים ראוי להיות איש
Next day Moshe came across two Jews fighting among themselves:
וַ֠יֹּ֠אמֶר מִ֣י שָֽׂמְךָ֞ לְאִ֨ישׁ שַׂ֤ר וְשֹׁפֵט֙ עָלֵ֔ינוּ הַלְהׇרְגֵ֙נִי֙ אַתָּ֣ה אֹמֵ֔ר כַּאֲשֶׁ֥ר הָרַ֖גְתָּ אֶת־הַמִּצְרִ֑י וַיִּירָ֤א מֹשֶׁה֙ וַיֹּאמַ֔ר אָכֵ֖ן נוֹדַ֥ע הַדָּבָֽר
Rashi comments: נודע לי דבר שהייתי תמה עליו: מה חטאו ישראל מכל שבעים אומות להיות נרדים בעבודת פרך, אבל רואה אני שהם ראוים לכך.
Moshe now saw that it wasn't because of their nature as rachmanim that the Jewish onlookers avoid the conflict, v'ha'raayah, they are busy fighting with each other! What happened to the כל אחד נותן נפשו על אחיו that the Ohr haChaim described? What happened to the ahavas yisrael?
אָכֵ֖ן נוֹדַ֥ע הַדָּבָֽר why Bn"Y were in the situation they were in.
Still, we can ask why seeing just these two people alone trading blows changed Moshe's perspective. הָאִ֤ישׁ אֶחָד֙ יֶחֱטָ֔א וְעַ֥ל כׇּל־הָעֵדָ֖ה תִּקְצֹֽף? Who says these two people are necessarily representative of the entire nation?
I have seen three answers to this question:
1) Midos ra'os are like a contagious virus. The problem won't stay with just two people -- it will inevitable rub off on and spread to others (R' Gershon Edelstein).
2) The fact that two people behave in such a way is indicative of the fact that the community has a whole has a lax attitude towards the issue (to some degree or other). Let me give you an example. When I was growing up, I don't think any kid wearing a yalmuka would ride a scooter on Shabbos, certainly not to shul. It just wasn't done. Recently my wife and I were walking on Shabbos and in front of one shul it looked like they needed a parking lot for the amount of scooters in front of the building. Our attitude towards Shabbos has changed. The kid riding the scooter didn't cause the change; he/she is just a siman. When we were kids we knew it wasn't done; now kids don't have that same innate sense that this is something off limits. I can say the same about our attitude towards kedushas beis ha'knesses -- does anyone think twice anymore about walking into minyan with a cup of coffee and cell phone out? -- or any number of things. If there was a feeling that this was completely out of the norm of what our communal standards are, people would be far less likely to do it. People no longer feel that way. Moshe thought the same about the infighting that he witnessed. If this was truly way beyond the pale of accepted behavior, it would never have happened. The fact that it did showed that something in the bigger picture was off (R' Shteinman).
3) Once you get involved in breaking down the community between "those guys" who engage in the wrong behavior vs the rest of us, m'meila there is no longer a united klal, and Moshe understood that geulah can only come to the klal in the zechus of the klal. (R' Shteinman)