John Calvin Commentary Numbers 1:20

"And the children of Reuben, Israel`s first-born, their generations, by their families, by their fathers` houses, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war;" — Numbers 1:20 (ASV)

And the children of Reuben, Israel’s eldest son. If any contentious person should argue that one family could not increase to such a large number in 250 years, and therefore reject as fanciful anything that surpasses the ordinary rules of nature, we must remember what I have already stated: since this increase depended on God's power, it is utterly absurd to measure it by ordinary standards. For the Spirit's intention is to display before our eyes the incredible power of God through a clear and significant miracle.

Meanwhile, if you compare the tribe of Reuben with some of the others, its numbers show some marks of the curse, so that we may infer that Reuben was degraded from the honors of his primogeniture. For the tribes of Simeon, Issachar, Zebulun, Dan, and Naphtali were more numerous, while from Joseph alone, who was one of the youngest, descended a posterity that almost doubled Reuben's numbers.

God’s blessing, however, is most evident in the tribe of Judah, in keeping with Jacob's prophecy. For by this prerogative, so to speak, Judah was already called to the right of primogeniture and to supremacy, since it surpassed all the leading tribes.