A more than $140 billion tie-up between Unilever and Kraft Heinz would be the biggest U.S. acquisition of a foreign company and the largest in the consumer staples sector, said Richard Peterson, senior director at S&P Global Market Intelligence. From a global perspective, the proposed deal would be the third largest on record without considering the value of corporate investments, he said.
"3G and Berkshire, they're noted for trying to find efficiencies, and this is on a massive scale," Peterson said.
The two investor groups together own just over half of Kraft Heinz. As of December, Berkshire had a 26.7 percent stake in the food company's outstanding shares and 3G owned 23.9 percent, according to S&P.
Spokespeople for the Buffett-led conglomerate and 3G did not immediately respond to a CNBC request for comment.
Hellmann's mayonnaise owner Unilever rejected Kraft Heinz's offer Friday, saying the proposal "fundamentally undervalues" the Anglo-Dutch consumer goods giant. Kraft Heinz said in a statement that it still looks forward to reaching an agreement on a deal, but added that it's uncertain whether another offer will be made.
Kraft Heinz itself is the product of a Berkshire-3G effort.
In 2015, Berkshire and 3G backed Heinz in its roughly $45 billion merger with Kraft Foods, creating the third-largest North American food company. Kraft Heinz Chairman Alex Behring is a managing partner at 3G Capital, which he founded. Behring was formerly the chairman of Heinz.
In December, reports surfaced that Kraft Heinz was planning to acquire Mondelez International. At the time, Reuters said, citing sources, Mondelez had not heard from Kraft-Heinz. Mondelez told CNBC then that it does not comment on market rumors or speculations.
— Reuters and CNBC's Antonio José Vielma contributed to this report.