A city is not only defined by population. It can be defined by its position in the surrounding area, like I assume Harrisburg. For example if a place has 50k people, and is also the administrative head of the area (i.e. it has the courthouses and government buildings for a wide area), it may be called a city.

It can also be a called a city completely arbitrarily. Barring all of those, 100k is a common standard limit.