Explain why urban centres based mainly on transportation are usually quite small in size.

Although cities played a significant role throughout human history, it is not until the industrial revolution that a network of large cities started to emerge in the most economically advanced parts of the world. Since 1950, the world’s urban population has more than doubled, to reach nearly 3.8 billion in 2014, about 54% of the global population. This transition is expected to go on well into the second half of the 21st century, a trend reflected in the growing size of cities and in the increasing proportion of the urbanized population. Cities also dominate the national economic output as they account for the bulk of the production, distribution and consumption. Urban mobility problems have increased proportionally, and in some cases exponentially, with urbanization since mobility demands are concentrated over a specific area.