It is relatively compact, with sky-rise apartments funneling people into the same paths of commerce and transport. So you end up seeing the same people and shopping at the same stores. In that sense, it feels like a small town - especially as an expat. Also, outside of the downtown areas, you don't get the swarming hordes of people that megatropolises like Seoul shove at you. Mostly you can just go about your business
However, Cheonan with 600,000 people is large enough to avoid the feelings of isolation that come with living in the countryside. There are plenty of good stores, restaurants, public transportation, and other conveniences.
Cheonan has been my home for the past nine years, and I'm going to miss it.