According to them, all bishops are actually equal, so it shouldn’t matter how many patriarchates each side has.
In any event, those five Patriarchates didn’t represent equal portions of the Church–it’s wrong to say 4 of the 5 eastern patriarchates at the time of the schism were 4/5s of the Church. By divine institution, there is only the order of bishop with all being equal except Rome having primacy. Through positive law and custom, Alexandria and Antioch acquired additional authority based on their Petrine ties. These three Patriarchates, Rome, Alexandria, and Antioch–in that order of primacy–governed Europe, Africa, and Asia, respectively, with Rome as the final court of appeal.
As early as the First Council of Constantinople, the emperor tried to get the bishop of his city to replace Alexandria as the Eastern primate. This was opposed by Pope St. Damasus, who reiterated the traditional ordering. This was tried again at Chalcedon, but again, the Roman Pope (St. Leo the Great) vetoed it. However, when Alexandria rejected the definitions of Chalcedon and separated from the Church (leaving only Rome and Antioch), Constantinople filled the void and the Greek Patriarchate of Alexandria was instituted and was essentially a vassal of Constantinople. Jerusalem was was also separated from the jurisdiction of Antioch at Chalcedon and fell more under the influence of Constantinople, especially when the emperor later made it a patriarchate. The rise of Islam also significantly weakened Antioch to the point where it became significantly reliant on Constantinople.
So at the schism, you really only had the separation of two original Patriarchates–Rome and Antioch–and Antioch paled in significance at the time. If you prefer, it can also be looked at as a split between the chief See of the Church and the imperial power in the East, which held significant influence over the other eastern churches.
I also think the EO lack two notes of the Church in the Creed. I just did a long post on this in another thread:
Orthodox Churches versus Roman Catholic Apologetics
The Eastern Orthodox Churches lack two of the notes of the Church we profess in the Creed: they are not “one” nor are they “catholic.” These notes are interelated (you can’t really have one without the other). The jurisdiction of the papacy is necessary for the Church to be one. The Eastern Orthodox Churches are a perfect example of why this is. They get into situations where EO Church A is in communion with B, B is in communion with C, but A and C are not in communion with each other (A=B=C≠A)…