A rally that has propelled U.S. equities to record highs increasingly rests on red-hot chipmaker Nvidia and a handful of other giant stocks, reviving concerns that the market’s performance has become tied to a cluster of companies. A look at the ten largest stocks in the S&P 500 shows their weighting ballooned to 34.1% at the end of May, the highest-ever month-end weight for the index’s top ten. Many investors believe the companies’ market heft is deserved, given their robust earnings, dominant competitive positions and expectation they will capitalize on advances in the burgeoning artificial-intelligence field. But some are concerned the concentration of gains in a handful of powerhouses could threaten indexes if some of the big names start to wobble.