Albert Barnes Commentary 2 Chronicles 13:3

"And Abijah joined battle with an army of valiant men of war, even four hundred thousand chosen men: and Jeroboam set the battle in array against him with eight hundred thousand chosen men, who were mighty men of valor." — 2 Chronicles 13:3 (ASV)

Some have proposed changing the numbers here and in 2 Chronicles 13:17 to 40,000, 80,000, and 50,000, respectively. This suggestion is based partly on these smaller numbers appearing in many early editions of the Vulgate, but mainly because the larger figures are considered incredible.

However, the numbers are consistent with the census taken during David's reign (1 Chronicles 21:5), especially when combined with the fact, which the writer previously related, of a significant subsequent migration from the northern kingdom into the southern one (2 Chronicles 11:13–17).

At the time of the census, the total adult male population was 1,570,000, while the total number of fighting men now is 1,200,000. This would leave a remainder of 370,000 for the aged and infirm, which is nearly a fourth of the whole.

Furthermore, the loss mentioned in 2 Chronicles 13:17 can be understood to mean the entire Israelite loss over the course of the war, which likely continued throughout the whole reign of Abijah.