San Juan metro area accounts for more than 3 quarters of the population of Puerto Rico. Same with the Kuwait City metro area in Kuwait.
What: Population distribution between the largest city, the urban area without the largest city, and the rural population. Metro area is meant by the largest city in most cases.
Where: Top ranking countries except one-city countries (like Singapore) from the whole world. Some countries with a total of 26 million inhabitants were excepted because they do not have needed statistics in the Worldbank database. Many of them are small, possibly one-city countries.
Source: Worldbank for numbers, Wikipedia for names.