And that runs counter to the very purposes of the public markets.

“When a company goes public they’re available to all investors to take benefit of that growth,” Cunningham said on Tuesday at Fortune‘s Most Powerful Women conference in Laguna Niguel, Calif.

Cunningham said that the number of public companies has fallen by about half in 20 years because of a multitude of factors, including the availability of massive amounts of private capital. But she also said that compliance requirements and other requirements such as those governing how to communicate with shareholders, were leading many companies to wait to be of a certain size so they could afford and support such units before listing on the stock market.

“If you wait for a company to become a really big company then most of that growth has already happened, so the everyday investor misses out,” she said.

Cunningham became the first female leader in the 226-year history of the NYSE, a unit of Intercontinental Exchange (ICE), in May, about a year and a half after archival Nasdaq OMX (NDAQ) also named a woman to lead it, CEO Adena Friedman.

Both face enormous pressure on the business as stock trading continues to shift to different marketplaces- some 40% of stock trading now occurs off of stock exchange, a far cry from the NYSE-Nasdaq duopoly of only a few years ago. The two exchanges fight hard for new listings. Some of NYSE’s big wins this year include Spotify (SPOT), BJ’s Wholesale Club (BJ) and AXA Equitable Holdings.

Defending Wall Street, which she said “often gets a pretty bum rap,” Cunningham said flourishing exchanges were key to helping companies thrive. “It (NYSE’s core mission) is to raise money so they can go out and change the world and to provide investors with the opportunity to participate in that growth,” she said.