without facts, I would expect some central Asian horse army to have 200,000 humans, far earlier than that.. Persian and Egyptian armies were large at different times too.. sounds uninformed..
The OP says largest army at the time, not largest ever
Really? In 1865? Which one of those was in existence and larger?
Asia has held the bulk of human population for quite a long time.
The Qing dynasty (i.e., China), which had just put down a massive rebellion, probably had many times that number of men under arms.
As far as I know Qing military organization was weird for the time. There was no single central Chinese Army as such. There were some forces which officially reported to the Imperial government (the Green Standard and the Eight Banners), but these were of limited use. The armies that put down the Taiping Rebellion were raised by local governors, and were loyal to them not to the Emperor.