Why did the southern kingdom eventually have more troops than the northern kingdom?
Over the intervening years, a mass migration of Israelites out of the northern kingdom into the southern kingdom caused the shift. The northern kingdom’s ten tribes dwindled to 580,000 troops, while the southern kingdom grew to 1,160,000, completely reversing the earlier numerical advantage.
Initially, Abijah of the southern kingdom of Judah and Benjamin could raise only 400,000 top troops, while Jeroboam of the northern kingdom set 800,000 against him. That was logical because the northern kingdom had ten tribes and the southern kingdom had only two. But as the numbers are followed through the intervening years of history, the northern kingdom can finally put only 580,000 troops in the field, whereas the southern kingdom comes on the scene with 1,160,000. Les explains that this happened because of a mass migration of Israelites out of the northern kingdom down into the southern kingdom. Over the years they kept filtering down and filtering down, so that the tables were completely turned.
And Abijah set the battle in array with an army of valiant men of war, even four hundred thousand chosen men: Jeroboam also set the battle in array against him with eight hundred thousand chosen men, being mighty men of valour.