I thought the same initially but this may just be a case of recency bias. Small caps have underperformed large cap stocks for the last ~12 years but these things tend to go in cycles: https://blogs.cfainstitute.org/investor/2025/04/24/small-cap....

It will be interesting to see how the next cycle plays out with the recent concentration of returns in large cap tech stocks (Magnificent 7).

Looks like that was a one-time event in the 70s. The rest of the time, the small stocks line goes roughly parallel to the large stocks line.

In finance there is an ongoing discussion of whether the small-cap premium still exists. For technical discussions, look for the terms "SMB size factor".

And it's not obvious thanks to the logarithmic scale... There must have been a better way to present this information.