The concentration of these Mag 7 stocks—now one-third of the U.S. market—gets talked about a lot because one-half of the S&P 500’s climb over the past two years can be attributed to them.

Several members of the Mag 7 were falling along with the broader stock market on Tuesday as Wall Street digested potential risks from tariffs. The Roundhill Magnificent Seven ETF was off 1.1% on ...