Gauging a city based on metro vs city boundaries

I have noticed there are forumers who still insist in using city boundaries to gauge a city. My question is does gauging a city based on its city boundary have any foundation? Most cities have outgrown their city boundaries so there is no basis why some people still insist in using the boundaries. The boundaries are not modified just because there will be a lot of bureaucracy involved in changing the city boundaries and not for any other reason. Could the forumers who insist on using city boundaries explain why they do this?>