Certainly, if Qin Island City solely consisted of its urban area, the size wouldn't be large. The primary reason for its vast scale is that the districts under its jurisdiction are quite substantial, often exceeding thousands of square kilometers. For instance, just the Huangdao District alone is as large as Japan, Tokyo, covering 2,100 square kilometers.
That is to say, although Zhang Yanhang can produce a fairly large quantity of thermo-baric bombs, he's probably still a long way from being a 'city-level' power.
Okay, perhaps comparing Qin Island City, with its population surpassing twenty million, to a city-level power isn't quite appropriate. After all, cities of such magnitude are extremely rare within the Empire, let alone the entire world.
More common are the ordinary large cities with populations only in the millions. For those cities, Zhang Yanhang could now destroy them without much difficulty.
