Goldman Sachs strategists warn that even a minor 1% rotation of S&P 500 market cap into the Russell 2000 would represent a massive 19% of the small-cap index's total value. This stark disparity highlights the outsized impact flows can have on smaller stocks, even as the broader market shows modest gains in early 2026.

The call comes amid a noticeable market broadening, with the equal-weight S&P 500 up 4% this year, reflecting improving performance beyond mega-cap leaders. Goldman expects this trend to continue in the near term, fueled by a strong economic backdrop and a supportive Federal Reserve.

However, the bank cautions that this rotation has limited runway. With U.S. GDP growth projected to cool in the second half and valuations for the average S&P stock already elevated, sustained broadening will depend on earnings growth spreading beyond the largest names. The upcoming 4Q earnings season will be a key test.