The S&P 500 has 500 companies, but a relatively small group is driving an increasing share of the index.
Apple and Nvidia now account for more than 15% of the S&P 500, while the 10 largest holdings are approaching 40%. A decade ago, the top 10 were around 17%.
Apple is up about 25% YTD and Nvidia about 21%, which has increased their weight as their market caps have grown.
That doesn't automatically make the concentration good or bad. It does mean that movements in a relatively small number of mega-cap companies can have a larger effect on the index.
For investors tracking the broader market, I'm watching whether performance starts spreading further across the rest of the S&P 500, or remains concentrated in the largest names.