The Russell 2000 index of small-company stocks has had quite a year so far, notching its greatest gains since 1991.

The index, which did its annual rebalancing at the end of last week, was also beating the more famous S&P 500 by nearly 14 percentage points through Friday, also its biggest margin in 35 years.

But things might be about to change. As part of the index rebalancing, 43 companies-including some of the index's best performers-will graduate to the Russell 1000 large-cap index.

The Russell has clocked double-digit gains prior to its mid-year rebalancing two other times this decade, returning 16% in 2019 and 18% in 2021. Both times the second half of the year was decidedly less spectacular, with a 6.5% gain in 2019 and a 3.8% loss in 2021.