Of course we never did before. But then again, the American Empire used to be just the Enclave, which didn't have all that many people if I'm not mistaken. We all started small, but we grew. Why would the BoS be any different? For another example, the USNR used to be just New Vegas, but then it expanded and grew. The Brotherhood has also done that. Like I said, it's not unreasonable for us to be that big.
Well, the KOT, Legion, and ROA controls large amounts of lands. OH I think would be near 700,000-800,000. NEC would be around 100,000-400,000. BOS would be around our levels. UNSR since with there breeding programs would be on similar BOS and OH levels. 2AE is on the smae level as us, NCR would around 900,000. PRB would be the smallest, 200,000.