Europe has plenty of very big companies. They are just not in tech (as in consumer computing, social media).
Or the large companies that are repackaged versions of much older companies, with lineage back to the 19th century. Exxon & Chevron, for example, trace back to Standard Oil (1870); AT&T and Verizon (from Bell, 1877), Dow/DuPont (1897 or 1802, take your pick), Citigroup (from Citicorp, 1812). Banking often goes further back: BofA (Massachusetts Bank, 1784), JPMorgan (Manhattan Company, 1799).
Moving cuts of the pie around is a shell game. For sure the US is less regulated than Europe, so labels change more often, but the old money doesn't.
Top? The S&P 500 is riddled with mediocrities and rent-seekers.
17.5%? Five out of five hundred is 1%.
17.5% of the market share.