well, whats the big deal then. those ships often trade off a small size for less firepower. - an osiris slips well through asteroids, a juggernaut is a nightmare there.
the problem is not a balance between battleships much. - an osiris would prolly mount 4 antifighter turrets, 1 heavy missile and then 7 primaries - and would still have a lot more firepower than a cruiser. - a juggernaut would have more firepower, no doubt, but why should an osiris go one on one vs. one of the big battleships anyway. they are not meant to be equal in terms of firepower - but equal in terms of features.
but i am biased maybe, cause the only BB i fly is the jug with 16 turrets.