Secondly, per your question, it's perfectly natural to expect economic activity to increase with population density. Increased density of people means that travel costs (both time and money) are far lower, and thus the profitability bar for niche goods and services is much lower. That means there's a lot more demand that can be profitably satisfied given the population and their skills, and thus they can be better employed.

In other words, there are reasons cities increase the expectation of employment: they lower transaction costs and increase economies of scale.