What is the difference betwixt a "megalopolis" and a "metro statistical area"?

"Megalopolis" is a word used for descriptive purposes, but it's not an official designation. It means a huge, sprawling city that takes up a lot of land. On the other had, even a small city has a "metro statistical area".  There have to be various ways of describing a city. For example, Atlanta has a relatively small area within the city limits, and most of the area is occupied by office buildings and stores, not by homes. So the number of people actually living in Atlanta, if you're talking about the city limits, is pretty small- about 150,000 people. BUT millions more live in the surrounding areas, work in the city, use the services of the city, make the traffic a nightmare- for evaluating many things, those people have to be included in the statistics. 

megalopolis refers to the largest, likely sprawling, cities.  an MSA is a statistical contrivance of the government to ease of thinking.  Example:  Shanghai is a megalopolis.  Omaha, Nebraska is the center of an MSA; so are Portland, Maine and Plattsburg, NY.