Even today, Jerusalem is not that big geographically, but I suspect in the first century its population was more than a few hundred. The Great Revolt there claimed hundreds of thousand of lives. Tacitus estimated the casualties from this clash at 600,000 (Histories V, 13). Josephus estimated them at 1.1 million (Wars 6.9.3). Some current estimates determine the casualties to be at more than 1.3 million, an estimate that includes civilians such as moderate Jews visiting the city to celebrate Passover.
The casualties included Romans, too, but a couple hundred Jewish soldiers could not have killed a million Roman soldiers.