Share your insights or perspective on the financial matter presented in the input.

What evidence or research suggests that mid- or small-capitalization stocks should perform better than large caps?
From Dimson, Elroy, Paul Marsh, and Mike Staunton. Triumph of the Optimists: 101 Years of Global Investment Returns. Princeton, N.J: Princeton University Press, 2002: Disappointingly, the small firm effect has not proved the road to   great riches since soon after its discovery, the US size premium went   into reverse. This was repeated in the United Kingdom and virtually   all other markets around the world. Despite their disappointing performance in recent years, the very   long-run record of small-caps remains one of outperformance in both   the United States and the United Kingdom. Furthermore, mid- and   small-size companies are still an important asset class. Their   differential performance over long periods of history shows that there   is useful scope for investors to reduce risk by diversifying across   the “large” and the “small” capitalization sectors of the market.   Furthermore, given the pervasiveness of the size effect across the   entire size spectrum, it is important to all investors since the   size tilt of any portfolio will strongly influence its short- and   long-run performance. This holds true whether there is a size premium   or a size discount. The size effect has certainly proved persistent   and robust. What is at issue is whether we should continue to expect a   size premium over the longer haul. And accompanying charts:    And one chart from BlackRock: