Let me rephrase your question, Jcooper, in a way that might explain the answer:
Why would a person put a fire out with one pilot when he could employ 9?
The science of Operations analysis helps determine the best ways to answer such operational questions as How Big? How Many? What capabilities?
In this case, the critical mass of water is one requirement, and the fire determines that. Dousing enough area to stop the fire from jumping the wet spot is a real issue.
Besides that, the cost of the system goes down with size, which is usually based on the gross weight of the aircraft, and the cost of crewing it, which is a per-person cost.
After purchase price, the people needed to run the system are the biggest expense. IN fact, the two are about equal when we divide the price out over the life of the vehicle. That's why supertankers were invented, why modern airliners have 300 to 700 seats, and why double and triple trucks are making their appearance. Fewer crew per passenger or ton of cargo.