Southeast Asia includes territories situated on the main Asian continent as well as in the Pacific ocean, so it is best to travel by air than by land or water. It is also relatively cheap due to the availability of local and regional low-cost airlines. In addition, flight times between the farthest main cities are no more than 5 hours.
The top 5 largest countries by land area and population are:
By land area: Indonesia, Myanmar, Thailand, Vietnam and Malaysia;
By population: Indonesia, Philippines, Vietnam, Thailand and Myanmar.
Singapore is a city state, thus has a 100% urbanization rate. Brunei and Malaysia’s urbanization rate rank 2nd and 3rd, but could still be considered rather high. The reason for Brunei is it being a very small but affluent country, while for Malaysia is likely from it being comprised of multiple states with their own capital cities and heads of state. Indonesia and Thailand’s rate of urbanization are close to the global average and are similar in how their populations huddle around a few large cities with large industries such as manufacturing, trade and tourism. As for the other countries, the urbanization rates are not so high most likely because of agriculture being their main industry.