As of Friday morning, the small-cap index has climbed 5.4 percent, compared to a meager 2.7 percent gain for the S&P. That said, this follows a period of substantial outperformance for the large caps. The S&P 500 rose more than 13 percent in 2014, while the Russell was up less than 5 percent.
"I think part of this is just mean reversion, because the Russell has lagged so far for so long that it's natural just to catch up," said Boris Schlossberg, a currency strategist at BK Asset Management, in an interview with CNBC's "Trading Nation."
"A lot of the flows that supported the big-cap stocks, like dividend chasing, has kind of priced almost all of the value out of the larger stocks, so investors are now looking for lower-priced stocks in the Russell 2000."
But it's not just that investors are springing for the last available assets. Schlossberg says the surge in small caps could also reflect a bullish economic thesis.
"The idea here is that the U.S. economy is going to finally start to really accelerate growth, which definitely favors small-cap stocks going forward," the strategist said.