I think what keeps the balance is that the CM have no limit to their numbers (i.e. no codex Astartes) and have more ways of recruitung more marines i.e. fabius, normal implantation with either CM or SP gene-seed , reserections and Daemonculabas.
However, with the constant wars in the eye taking their toll and rampant mutation ruining good gene-seed, tehy would be considerably slowed to the point where their numbers are fluctuating but within constant boundries.
That been said, the Black Legion was numerically inferior at one point and near extinction and now has ten times the men of the Word Bearers. Even with recruits from other legions and renegades an increase of that scale would need the creation of new marines on the scale only a legion could manage.