Is the trade growing parallell to the increase in population?

I have a town with close to 1000 inhabitants. My impression is that the economy (costs) increases almost exponentially as the population increases. (A large part of the cost are defense towers. I find that there are a need for a lot of those. As the area of the town increases and hase to diverge for mountains and lakes, it becomes increasingly more difficult to defend.) But I don’t see the trade increase in the same mannor? My city’s economy is dependent on trade. If the trade does not increase it becomes difficult to increase population size. More trade?