The minuscule tribe
כל-פקודי הלוים אשר פקד משה ואהרן על-פי יקוק למשפחתם כל-זכר מבן-חדש ומעלה שנים ועשרים אלף
The entire count of the Leviim, which Moshe and Aharon counted through the command of Hashem, according to their families, all the males from one month of age and above was 22,000
When the national census of the males was conducted, most of the twelve tribes were counted from the age of twenty. However, the Levite tribe was counted from the age of one month. Nevertheless, their total was less than any other tribe, not even reaching half of the smallest tribe. Why was this so? One suggestion is because they were the only tribe in Egypt that wasn’t enslaved. The Egyptians embittered the Jews’ lives, and the more they tortured them, the more Hashem had them multiply. Jews were miraculously having as many as six babies at once. Since the Leviim weren’t enslaved or tortured, they didn’t receive this miracle growth to their population.
However, it’s possible to propose another explanation. Chazal teach us one reason why the Egyptians were struck with the plague of blood is because they prevented the Jewish women from spiritually purifying themselves after their period in a mikvah. As well, we are taught that the terror from their Egyptian overlords caused the Jewish women to not have their period. What this means is they never had a point where they were forbidden to their husbands. As a result, they were freely able to engage in procreation.
However, the tribe of Levi didn’t feel this terror. They didn’t have any oppressors. The women therefore needed to go to the mikveh like normal. The problem was, the Egyptians prevented the women from going to the mikveh. Only the really pious were able to sneak out and find a source of water to immerse in. Everyone else remained forbidden to their husbands. This meant that they weren’t able to procreate properly.
Although the rest of the Jewish women didn’t have their period, Hashem performed a miracle and they were able to conceive regardless. As well, as already stated, their subjugation resulted not in their obliteration, but in their multiplication. They were able to exponentially grow during their time in Egypt. This was a result of the terror from their oppressors, which the Leviim were lacking. As such, the latter they weren’t able to procreate. They therefore remained the smallest of the tribes.
Good Shabbos and Chag Sameach!
 Based on Be’er Yosef to Numbers 3:39
 Numbers loc. cit.
 Ramban ad. loc. v. 14
 Shemos Rabbah 5:16, brought by Rashi to Exodus 5:4
 Exodus 1:12
 Midrash Tanchuma Shemos § 5, brought by Rashi to Exodus 1:7
 Ramban loc. cit. Cf. Ohr HaChaim and Keli Yakar loc. cit.
 Shemos Rabbah 9:10
 Midrash Tanchuma Metzora § 9
 See Niddah 16a
 The idea of the lack of their periods leading to an increase in children also appears in Sha’ar HaLikkutim parshas Shemos