If it's not the volume of the tail tank, then it must be the volume of some other tank - the way it's working (decreasing mass with decreasing SC) means it must be a volume constraint somewhere. How about this?:
The tail tank mass is balancing the mass in some other tank forward. That forward tank is full. Therefore the mass in the forward tank falls as SG drops, therefore the mass you (need/can have) in the aft (tail) tank also falls.