What is the smallest big city in America?

I'm curious to know, could you please clarify what is meant by "the smallest big city" in the context of America? Are we referring to a city that is still considered large but has a relatively small population compared to other major metropolises? Or perhaps, is there a specific criterion that defines a city as "big" while also being the smallest among its peers? Additionally, how does one go about determining such a classification and what factors might contribute to a city being labeled as such?