In what has been a banner year for mid-cap stocks and exchange traded funds, there is no dearth of ETF winners from this group. That includes the $12.8 billion iShares Russell Mid-cap ETF (NYSEArca: IWR), which is up 3.7% year-to-date, topping the S&P 500 by nearly 130 basis points in the process.

The rotation into mid-caps is fueled by perceived currency risks that large-caps are exposed to – large multi-national U.S. corporations may see revenue streams diminish as overseas currencies continue to depreciate against the U.S. dollar. [Mid-Cap ETFs Take the Lead]

Although the dollar has recently pulled back, that does not diminish the allure of IWR, S&P Capital IQ’s focus ETF for May.

IWR “is in the top quartile of our equity ETF ranking universe of approximately 800 products, based on a combination of performance analytics, risk considerations and cost factors,” according to the research firm. “We believe mid-caps have benefitted relative to large-caps as they have a lower concentration of revenues from international operations that are hurt by the strong U.S. dollar. Meanwhile, the mid-cap index has different sector exposure than its larger alternative.”

IWR, which tracks the Russell MidCap Index, offers investors a deep bench approach with 841 stocks, none of which accounts for more than 0.53% of the ETF’s weight. Just three of IWR’s top 10 holdings garner weights north of 0.5%. Six sectors are represented among IWR’s top 10 holdings, a group that combines for just over 4.5% of the fund’s weight.

Mid- and small-cap companies tend to be U.S. focused and are more closely tided to the well being of the U.S. economy. With the U.S. economy on the rise and low energy prices fueling growth, mid-caps could continue outperforming.[Mid-Cap ETFs for Growth]