The opportunities you get in a city of 3million is different from 600k is different from 250k. I've lived in all of the above over my life. You can find niches in each. However the larger the city the more options. The smallest city had great Thai food - some family from Thailand moved there and opened a restaurant, but there was no Vietnamese, the next largest city has both, and the 3 million city more options than I was ever able to check out. (there are cities > 10 million around with even more options).