Be’er Yosef: Ramban2 asked the question first. The population figure in this pasuk does not come to half of the similar age group in the smallest of the other shevatim! Why was shevet Levi depressed in its population? Some of the factors that might hold down the numbers of Leviim did not even apply at this point. While it is true that the kedushah of the kelim of the mishkan could prove fatal to any Levi carrying them improperly, the total in our pasuk reflected a shevet Levi before its service in the mishkan began. (Ohr HaChaim adds that in the aftermath of the Golden Calf, many lost their lives – but not anyone in shevet Levi, which did not take part in that transgression.)
Why would it be that the group of Jews most identified with genuine avodah of Hashem should not merit the blessing of children to the same degree as the rest of the nation?
Ramban’s opinion focuses on the Egyptian oppression. Shevet Levi was not enslaved. Its members did not suffer like the rest of the Jewish people. G-d responded to the pain of the Jewish people with a blessing for children. Those who did not suffer did not receive the berachah of unnatural population growth. The Leviim populated like any other group would – no more, and no less. Hence, in comparison with all the other shevatim, their numbers were modest.
We could point to another factor. Chazal3 show how the ten plagues in Egypt followed from the crimes of the Egyptians. The plague of blood was visited upon them because they prevented Jewish women from using the mikvah to rid themselves of the tumah of blood. Their purpose was to thus deny the Jews the opportunity to become pregnant and propagate. According to the Tanchuma, 4 however, the plot failed. The women did not evidence their usual monthly cycles, and never became forbidden to their husbands. Thus, to the contrary, there were no time-bound restrictions at all on their marital lives. This led to a sharp increase in their birth rate. (The gemara5 similarly posits that a state of fear will interfere with the monthly period. Thus, the Jewish women did not commonly experience their period that would have interfered with opportunities to consort with their husbands.)
All of the above, however, applied only to the other shevatim. They lived through the oppression and the terror of the Egyptian enslavement. Shevet Levi, on the other hand, knew nothing of this. Their physiological lives continued as usual. This meant that they were in need of mikva’os – and denied their use by edict of the Egyptians. They were able to immerse in a mikvah only through subterfuge. Their families for the most part suffered forced separation when they were not able to use the mikvah. This kept Levi’s population down.
It could be objected that women from all the other shevatim whose monthly cycles disappeared also became incapable of becoming pregnant! Ordinarily, this would be a typical consequence. It was not so, however, in regard to the Jewish women. Hashem kept them miraculously fecund. Without having to worry about mikvah, they were able to produce children abundantly – in far greater numbers than Shevet Levi.
1. Based on Be’er Yosef, Bamidbar 3:39
2. Bamidbar 3:14
3. Shemos Rabbah 9
4. Tanchuma, Metzora, end
5. Nidah 16A