The chart shows the change in the S&P 500's market capitalization, which is a good proxy for the returns calculated by S&P.
This "simple" market capitalization change is up 9.16% YTD.
3.58% of this is gain NVDA. That's 39% of the S&P 500's gain YTD!
AMZA, MSFT, and META account for another 1.41% of the S&P 500's gain YTD, or 15%.
54% of the S&P 500's gain YTD is just four stocks!
These four stocks account for just 19% of the total market capitalization for the S&P 500.