An Illinois town with a population of just 25,000 grew to roughly 250,000 after they hosted a fair that was record-breaking in attendance. Welcome to Belvidere!. Living in Rockford with a population of over 150,000, I don’t ever think about other surrounding cities because, in my mind, we’re all one giant city. How the heck did Belvidere, Illinois fit 250,000 people in one place?! Insane!