I don't think that's true. We speak of AppAmaGooBook now (i.e. an oligopoly of 4 major players), while in the 1990s it was "Wintel" (2 players, one a hardware monopoly and one a software monopoly) and in the 60s it was "IBM and the 7 dwarfs" (i.e. IBM was the only relevant computer company). IBM controlled 70 percent of the whole computer market, around what Google controls of the Search market now, which is a small subfield of computation. And these all pale in comparison to 19th century monopolies, like Standard Oil (90% of U.S. refining capacity in 1880) or American Tobacco (100% of the tobacco market in 1900).

I think the reason they seem bigger is because the economy is bigger, and so in dollar and output terms they are bigger. But that also means there are more niches and greater complexity, and it becomes harder to sustain a monopoly against multiple competitive threats over time. While the playbook of monopoly managers has expanded since 1900, the number of adversaries they face has expanded faster.