It's the largest purchase of Teslas ever by a single buyer, and the biggest move into electric vehicles by a rental car company. But even CEO Elon Musk was surprised by the magnitude of the jump, which pushed Tesla's market value up to $1.01 trillion. "Strange that moved valuation, as Tesla is very much a production ramp problem, not a demand problem," Musk said on Twitter. Nonetheless, Musk, whose net worth of nearly $289 billion makes him more valuable than ExxonMobil, cheered the move. "Wild $T1mes!" he boasted. Investors are in celebration mode, too. The S&P 500 and Dow both hit all-time highs on Monday, as Wall Street brushed off lingering concerns about a slowdown in economic growth, higher inflation and heavy debt loads in China's large real estate sector. But the growing $1 trillion club isn't necessarily a good thing for markets over the long run. Breaking it down: Five companies in the S&P 500 — Apple, Microsoft, Google parent Alphabet, Amazon and Tesla — are now worth a collective $9.3 trillion. That's almost 23% of the benchmark US stock index's total value. Add in Facebook, which is worth almost $927 billion, and the figure rises to 25%. In January 2020, Facebook (FB), Apple (AAPL), Amazon (AMZN), Microsoft (MSFT) and Google (GOOGL) accounted for 19% of the value of the S&P 500. That means that increasingly, a handful of firms have outsize influence over the index — and therefore the direction of financial markets. In boom times, that might not seem like such a bad thing. Big Tech companies (and now Tesla) have powered huge stock market gains since spring 2020 as they proved they could still earn billions of dollars during a pandemic. The stock market rally could get another boost this week, with earnings from Alphabet, Apple, Amazon and Microsoft on tap. But technology stocks are also very sensitive to changes in interest rates, which central banks are considering hiking to curb inflation. If Wall Street starts to dump shares of Apple or Amazon, that could generate a sizable pullback. The big question: Does market concentration equal market power? More than anything else, the answer could dictate the stock market's trajectory in the coming years, as policymakers take more aggressive steps to rein in the biggest technology firms, including through potential antitrust action.