Nobody can stay with certainty but the reason for the recent outperformance of the small caps relative to the large caps was most likely due to the rally in the US Dollar. Small cap companies typically derive most of their revenues from domestic (U.S.) sales while large-cap companies typically have a significant portion of their revenues coming from sales outside the U.S. (exports). As such, a rising dollar makes exports from the U.S. more expensive overseas & tends to hurt sales of large-cap stocks without as much of an impact on small caps.

One other possible reason is that the small caps, being more speculative & risky in nature than large caps, often experience a rally towards the end of a bull market as investor sentiment is high & retail investors often chase high-flying, aggressive stocks in the latter stage of a bull market. That part is yet to be seen of course & I would attribute a higher weighting to the rally in the dollar although both could be factors.