I can't answer your question for you, but I'd like to mention that it might help to define what you're looking for. Do you want to include sieges where an army surrounded a city to force surrender, but where one side (or both) had no desire to actually go to battle (e.g. Sparta's siege of Athens in the Peloponnesian War)? Are you looking for "largest" in terms of the number of attackers, the number of defenders, the combined forces of both sides, the total casualties, the total casualties including civilian casualties, or any other criteria? If based on troop counts, do you want to include reinforcements (e.g. the Battle of Alesia)? If based on casualties, do you want to include those from plague and starvation?

I'm interested in the answer as well, but the question may be a little difficult to answer at the moment, so maybe being a little more specific might help.

Something on a similar scale would be something like the Siege of Carthage. 80,000 Roman Soldiers Raze a city of 500,000 to the ground. Mongol Siege of Bagdad would also qualify, city of nearly a million (depending on your sources) besieged by over a hundred thousand mongols.

Generally when you look for these sorts of things you would look at battles that took place in countries with enormous populations, logistical capacity, and economic power. China does fit the bill, as does Ancient Rome. Just some food for thought.