Last month, Elizabeth Warren, the Democratic senator from Massachusetts running for president, grabbed attention with a proposal to curb the power of big tech companies by pushing them to reverse past acquisitions.

But large deals of the kind Sen. Warren targeted are rare among the Big Five—as Amazon, Apple, Facebook, Microsoft and Google-parent Alphabet are sometimes labeled—according to an analysis prepared for The Information by the research firm PitchBook. Meanwhile, smaller deals, which represent the vast majority of acquisitions by the companies, have declined in frequency over the past half-decade, the analysis shows. The Big Five have gotten big mostly by building new businesses organically, not by buying them.