Expansion costs money, running operations is cheaper. So when growth stops, profit happens almost by default.
Simple math. Suppose opening a city costs $10 (once) and returns $1/year in revenue.
Year 1: Open 1 city, loss = $10.
Year 2: Open 3 cities, loss = $29 (spending $30 on new cities, gaining $1 from year 1's city).
Year 3: Open 9 cities, loss = $86.
Year 4: expansion stops, profit = $13.