CHART: The Year-To-Date Returns For 15 Russell Stock Market Indices

In the U.S., the S&P 500 has been doing great. This index of large-cap stocks is up 19% since the beginning of the year.

The Russell 2000, a benchmark for small-cap stocks, has been doing even better. It's up 22%.

But the Russell 2000 is nothing compared to the Russell Microcap Growth Index. It's up 28%.

This latter index consists of tiny companies with a weighted average market cap of $543 million. The average price-to-book value is a high 3.65, but it's justified by a lofty 10% long-term expected earning per share growth rate.

"The Russell Microcap Growth Index maintains its number one spot for the sixth week in a row amongst the Russell-style indices with a gain of 28.29%," said Oppenheimer's John Stoltzfus.

With higher risk comes higher rewards.

Here's the year-to-date performance of 15 Russell indices via Stoltzfus.