Consider the fact that if the small UPS where to fail (and I think that is what happened), it will drop the load whether the utility power is there or not. The small UPS' are not arranged for dual inputs, so a small "in sereis" with the large will negate the benifit of the large, since it has no way of getting power to the computers if the small fails.
There are so arrangements in which series units are helpfull. Those are when the middle UPS has the ability of dual feeds and the large unit would be connected to the bypass of the small unit. So if the small unit where to fail to bypass, the big UPS would carry the load. I don't think this was your situation.
