The passuk reads:
לֹ֣א מֵֽרֻבְּכֶ֞ם מִכׇּל־הָֽעַמִּ֗ים חָשַׁ֧ק יְהֹוָ֛ה בָּכֶ֖ם וַיִּבְחַ֣ר בָּכֶ֑ם כִּֽי־אַתֶּ֥ם הַמְעַ֖ט מִכׇּל־הָעַמִּֽים׃
It is not because you are the most numerous of peoples that the LORD set His heart on you and chose you—indeed, you are the smallest of peoples;
Taking this passuk literally- the Bnei Yisrael are the smallest nation.
Further, Devarim 7:1 reads:
כִּ֤י יְבִֽיאֲךָ֙ יְהֹוָ֣ה אֱלֹהֶ֔יךָ אֶל־הָאָ֕רֶץ אֲשֶׁר־אַתָּ֥ה בָא־שָׁ֖מָּה לְרִשְׁתָּ֑הּ וְנָשַׁ֣ל גּֽוֹיִם־רַבִּ֣ים ׀ מִפָּנֶ֡יךָ הַֽחִתִּי֩ וְהַגִּרְגָּשִׁ֨י וְהָאֱמֹרִ֜י וְהַכְּנַעֲנִ֣י וְהַפְּרִזִּ֗י וְהַֽחִוִּי֙ וְהַיְבוּסִ֔י שִׁבְעָ֣ה גוֹיִ֔ם רַבִּ֥ים וַעֲצוּמִ֖ים מִמֶּֽךָּ׃
When the LORD your God brings you to the land that you are about to enter and possess, and He dislodges many nations before you—the Hittites, Girgashites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, seven nations much larger than you—
This is similarly backed up by the report of the spies, who relate how powerful the nations of Canaan seem in comparison.
And this is confirmed again in the beginning of Eikev:
כִּ֤י תֹאמַר֙ בִּלְבָ֣בְךָ֔ רַבִּ֛ים הַגּוֹיִ֥ם הָאֵ֖לֶּה מִמֶּ֑נִּי אֵיכָ֥ה אוּכַ֖ל לְהוֹרִישָֽׁם׃
Should you say to yourselves, “These nations are more numerous than we; how can we dispossess them?”
לֹ֥א תִירָ֖א מֵהֶ֑ם זָכֹ֣ר תִּזְכֹּ֗ר אֵ֤ת אֲשֶׁר־עָשָׂה֙ יְהֹוָ֣ה אֱלֹהֶ֔יךָ לְפַרְעֹ֖ה וּלְכׇל־מִצְרָֽיִם׃
Do not stand in dread of them, for the LORD your God is in your midst, a great and awesome God
However, the census records about 600,000 adult males. Assuming that gets us to a total population of around 3 million, it is problematic to assume that we were the smallest nation.
To illustrate this:
If we assume there are 70 nations, as per Bereshit Perek 10, that leads to a conservative world population estimate of 200 million.
Even if we are more restrictive, and do not use the notion of 70 nations, we run into problems. Explicitly in the Torah, Canaan contained 7 nations, which would put the population of Canaan at least 21 million, a number way beyond what a land so small could sustain without modern agriculture (Feel free to dispute this with data, but that is my understanding).
How do we reconcile both the fact that the Torah gives Bnei Yisrael a very large (for those times) population, and asserts in multiple places that they were indeed very small?