Analyst: Although US small and mid-cap stocks are up 20% year-to-date, caution is warranted regarding the bubble risk of "non-earnings driven" gains.

Year to date, the US small-cap benchmark index, the Russell 2000 Index, has risen 20%, poised to achieve its best performance since 2003, while S&P 500 Index blue-chip stocks have risen 10%, and the so-called "Magnificent Seven" tech stocks have gained less than 3%. Some analysts point out that the current rise in small and mid-cap stocks is largely driven by unprofitable components, with excessive optimism and bubbles still present, making sustainability questionable.