There could be historic rationale for nerfing larger artillery units- they arent meant to be used simultaneously. A battery that large would use a tremendous amount of ammunition and present a tempting target. Instead a portion would be placed nearby to replace damage as the battle raged.
I think thats also a reasonable excuse to plump up batteries in game. In a long fight like Fredericksburg where smoothbores are gonna be taking casualties, an oversized battery would be widdled down to maximum efficiency right when the fight is fiercest.