There are many other factors affecting the size of corporations. For example, a significant increase in the amount of regulation in other sectors of the economy.
More importantly, the raw number of corporations isn't how you measure a monopoly. Profit margins are the key sign of monopoly pricing- but in practice, corporate profit margins have been relatively stable in percentage terms over time. Prices for virtually all consumer goods are way down in real terms over the last 30 years.
In particular, trade might get you more competition with fewer corporations, but via competition with overseas producers. These might show up as a few importing corporations, representing a whole slew of competitors abroad.
More importantly, the raw number of corporations isn't how you measure a monopoly. Profit margins are the key sign of monopoly pricing- but in practice, corporate profit margins have been relatively stable in percentage terms over time. Prices for virtually all consumer goods are way down in real terms over the last 30 years.
In particular, trade might get you more competition with fewer corporations, but via competition with overseas producers. These might show up as a few importing corporations, representing a whole slew of competitors abroad.