The sizes of cities can be very different. This depends on the type of city. Cities built hundreds of years ago and which have not changed much are much smaller than modern cities. There are two main reasons. One reason is that old cities often have a city wall, and most of the city is inside it. Another important reason is that the streets in old cities are often narrow. If the city got too big, it was hard for a cart carrying food to get to the marketplace. People in cities need food, and the food always has to come from outside the city. 