This is because cyclically, large caps have done exceptionally well in the past 10 years or so. Many small cap stock indexes went up 300% in value between 2000-2012. During that same time, the S&P100 went up 20% in total.
Extend your graph back an extra 14 years and you'll see that the S&P500 is going to beat each of those indexes over the full 22 years.
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