For Rich Ross, technical analyst with Evercore ISI, the outperformance of small-cap growth is all about the American growth story. The U.S. is growing faster than much of the world, and both smaller stocks and growth stocks are more likely to be beneficiaries of American growth than lower-valued stocks and large companies, Ross said.
"I think this is a theme that we're going to see continue through the rest of the year," the technician said. "We continue to see outperformance by small over big and growth over value."
Kathy Lien of BK Asset Management agrees with Ross, pointing out that the two "primary stories" of 2015 have been the global uncertainty manufactured in Greece and China and the surge of the U.S. dollar.
Small cap growth stocks "are far more sheltered" from both of those factors, she said.
To those factors, Dennis Davitt of Harvest Volatility Advisors would add one more: The larger set of energy stocks in the S&P 500 than in the small-cap space.
"Despite what people say, oil keeps gushing out of the ground like zombies in a bad Brad Pitt movie," which is keeping a cap on big oil stocks, Davitt wrote to CNBC.