The world's biggest fund managers are giving shareholder activists the muscle to take on larger companies from Dell Inc. to Microsoft Corp.

“They're able to go after big companies because they are very effectively communicating with large institutional stockholders,” said Scott A. Barshay, a partner at the law firm Cravath, Swaine & Moore LLP, at the annual Bloomberg Markets 50 Summit in New York today. “That's a very new phenomena.”

Activist investors, who typically take small stakes in companies and lobby for higher dividends, stock buybacks or management changes, have also recently targeted Apple Inc., Yahoo! Inc. and Chesapeake Energy Corp. with their campaigns. The activist funds are increasing in size, and large mutual funds have shown an increased willingness to back shareholder proposals.