Jamieson, Fausset and Brown on 2 Chronicles 13:3

3. Abijah set the battle in array--that is, took the field and opened the campaign.

with ... four hundred thousand chosen men ... Jeroboam with eight hundred thousand--These are, doubtless, large numbers, considering the smallness of the two kingdoms. It must be borne in mind, however, that Oriental armies are mere mobs--vast numbers accompanying the camp in hope of plunder, so that the gross numbers described as going upon an Asiatic expedition are often far from denoting the exact number of fighting men. But in accounting for the large number of soldiers enlisted in the respective armies of Abijah and Jeroboam, there is no need of resorting to this mode of explanation; for we know by the census of David the immense number of the population that was capable of bearing arms (1Ch 21:5; compare 2Ch 14:8; 17:14).