If you are buying ETFs because you hope to build a more diversified portfolio than you believe you can get by purchasing individual stocks, this chart is for you. The chart below from Bespoke dispels the myth that ETFs always provide adequate diversification.
The chart shows the weighting of the top two holdings in each S&P 500 sector. Like many ETFs, the S&P 500 sector ETFs are market-capitalization-weighted.
As you can see, the top two stocks are driving the ship in most sectors. In the Communications Services, Energy, and Technology sectors, the top two stocks account for about 40% of each ETF.