12-pounders fired a heavier round which could cause more damage. 4-, 6-, and 8-pounders had twice the rate of fire of a 12-pounder for sustained firing
Some artillery officers argued that in field battles one 12-pounder was no better than one 8-pounder or 6-pounder, or maybe even worse. For example:
Allix, Système d’artillerie de campagne. Paris, 1827, p.147, 153.