That has supercharged the giants’ boom.

According to data from Goldman Sachs, the top 10 stocks in the S&P 500 this month accounted for roughly 27 percent of the total value of the index. That surpassed the previous peak, which came during the tech stock frenzy of the late 1990s. The top five companies alone — Microsoft, Apple, Amazon, Alphabet and Facebook — account for 20 percent of the index.

By the end of trading on Tuesday, all but Alphabet were up more than 20 percent since the market hit its recent low on March 23. Alphabet, the parent company of Google, was up 17 percent.

Their size gives these companies a lot of heft. All five closed lower on Tuesday, which helped drag the S&P 500 down 0.5 percent, even though 330 of the 500 companies in the index were higher.

While the concentration of market wealth in top companies has hit a peak in recent weeks, it has been a long-term trend in recent years, reflecting, in part, changes in the structure of corporate America. The years since the 2008 financial crisis have been marked by an increase in the consolidation of some industries, such as banking and airlines.

And as bigger companies have steadily grown, they’ve also snagged a larger share of profits. In 1975, the biggest 100 public companies in the country took in about 49 percent of the earnings of all public companies. Their piece of the pie grew to 84 percent by 2015, according to research from Kathleen M. Kahle, a finance professor at the University of Arizona, and René M. Stulz, an economist at Ohio State University.

The current surge of megacap stocks suggests that many investors expect those companies’ slices to expand even more after the current economic implosion, in which smaller companies are widely expected to be the hardest hit. Small-cap companies — such as those included in the Russell 2000 — carry more debt, making them vulnerable to having financing cut off by bond markets. They’re less globally diversified, making them heavily dependent on the performance of the economy in the United States, the current epicenter of the crisis.