I think he means that is what he thought when he was only 8 years old.
Its funny, when I was kid living in Queens, I had no idea how much bigger New York City was than most other cities. I cannot remember
any of the adults saying New York is bigger than XYZ city. Since in my family we were NY Mets fans, I thought NY was roughly similar to the Mets competition like Philadlephia, Cinncinatti and St Louis.