Big cities attract both the richest and poorest people to them in large numbers. The wealthy want to live there because businesses are there, as well as other city attractions. Poor people will want to live in big cities to find work. Therefore, the gap between rich and poor people will be very noticeable in large cities.
Yes, I believe the gap between the rich and poor is biggest in cities such as Miami, New York and Los Angeles. These larger cities have some of the largest businesses and highly compensated individuals in the country. Because of their size and population, these cities also contain a large number of fast food restaurants and lower income workers. I believe the gap is far more evident in these larger cities than anywhere else.
There are just as many poor in the country. In fact, in the country, being poor can go even more unnoticed, because there are fewer social agencies. There is a great deal of poverty in rural areas, and in rural areas, education and other social services are not as readily available. Poverty is a problem everywhere.
You can look at the income and asset gap between people on different sides of the same residential street, but the gap really is pronounced between the big cities and the rural areas. Housing prices are one of the major reasons, as more jobs and more businesses and more people are concentrated in the large areas. The gap is widespread across cities, states and countries of course.