The number of publically traded corporations used to grow with the US population as you might expect. More people means more customers and more workers which creates opportunities for more corporations. But the number of public corporations peaked in 1996 and today there are about half as many corporations on US stock exchanges as there were.
As a consequence of this trend, industries are more concentrated and the average company that has a listed stock is bigger, older, more profitable, and has a higher propensity to disburse cash to shareholders.
There are half as many corporations, but their average value has approximately quintupled, so the total size of the stock market has approximately doubled. In fact, the value of these companies (and their profits) have been growing much faster than the rest of the US economy. The entire stock market value was less than half of GDP in 1976 whereas today it is closer to 150% of total GDP.
As Jeff Sommer points out, there are a couple problems with this kind of increasing market concentration. As corporations get bigger, they get more monopoly (and monopsony) power which creates inefficiency and reduces economic growth. This also increases economic inequality and with less equal economic power comes less equal political power as corporations get more and more power to manipulate government relative to the median citizen.