According to The Kobeissi Letter, small-cap stocks continue to underperform as the Russell 2000's relative performance to the S&P 500 has reached its lowest point since January 2001. Over the past four years, the ratio between small-cap and large-cap stocks has declined by 36.0%. During this period, the S&P 500 has gained 41.8%, highlighting a significant disparity in performance that traders should consider when evaluating potential investments.