Tim Quast discusses why stocks could struggle to sustain a broadening rally beyond the largest names, such as those in the S&P 500. He says less than 10% of trading volume comes from fundamental stock picking, while models and high-frequency traders drive around 75% of volume. With 90% of assets concentrated in large caps and only 5% in the Russell 2000, Quast believes it's mathematically unlikely for the market to sustain a meaningful shift into smaller stocks.