Yes, Metropolis with only 1500 people is fairly small, but I don’t think that’s reason that people complain about pop growth.
Each time I play Train Fever (unless I have extreme financial problem so I cannot replace/add vehicles) I get 1000 people from the largest town before 1930 and 1500 before 1950. In the year of 1970, the largest town of my game reaches nearly 2500 people and the second largest town also have over 2000 people so game is virtually unplayable due to huge lag.
What’s different from my towns with other people’s town? maybe I play on medium difficulty? or other aspect affected on city growth?