Check out our recent CNBC video and article on an overlooked problem in small cap indices like the Russell 2000 and S&P 600: not enough tech exposure. The S&P 500 is 25% tech. Small caps are at 15-17%.
You can watch here: https://www.cnbc.com/2017/11/07/this-disparity-is-one-of-the-achilles-heels-of-indexing-strategist.html