In a big city, people can take the chaces to study and work best. There are many good University for u to choose in a big city. There you can express your ability to study what you like. Moreover, when you graduate, you also find it easier to find chances to get a good job with good salary, that is quite difficult in small cities or countrysides; because there concentrates many big companies and groups. Hence, many graduate students choose big cities to live and work. In short, it is where you can take advantages yourself.