By Anne VanderMey and Nicolas Rapp
October 5, 2012

FORTUNE — You may have heard it before: Only 19 Fortune 500 chiefs are women. That’s a sad statistic, but it doesn’t tell the whole story of women leaders in big business. Within the highest ranks of corporate America there are female execs running units so large that, if separated out from their parent company, they would easily score a place on Fortune’s list.

One example is Rosalind Brewer, who became the CEO of Sam’s Club this year. On its own, Sam’s is a $54 billion business. If it weren’t part of Wal-Mart (WMT), those revenues would make Brewer a Fortune 50 CEO.

At Fortune’s Most Powerful Women Summit this week, Brewer and fellow power player Michelle Gass from Starbucks (SBUX) talked with Fortune senior editor Jennifer Reingold about what it’s like to run two of the world’s largest businesses — and still have a boss.

Brewer says it’s not that different from being a regular CEO. “The good thing about Sam’s Club is that it’s run autonomously so I have the leverage to be entrepreneurial, to develop a culture within a culture,” she said. “But I do have a lot of respect for the overriding culture.”

Similarly, when Starbucks’s Howard Schultz asked Gass to take control of its operations in Europe, the Middle East and Africa, she said he put it this way: “It’s yours. Do what you want.”

That conversation was made possible, Gass said, by their trust and mutual respect — plus the unprecedented push at Starbucks for international expansion. “[Schultz] said, ‘I’m reorganizing the company, the future of Starbucks is really about international growth,’” Gass told the crowd of women gathered in Laguna Niguel. She said it was “the first time he shipped a direct report of his literally overseas.”

Of her boss, Wal-Mart CEO Mike Duke, Brewer says, “the relationship between Mike and me is an interesting one.” With five years at Wal-Mart, Brewer is a relative newcomer to the company, and Duke looks to her for an outsider’s perspective. And they spend a large chunk of time together. The executive council meets monthly for a full week — Monday at 8 a.m. through Saturday at noon. Brewer and Duke also have one-on-one sessions every two weeks. Says Brewer: “He does not miss those.” They conference even if Duke is out of the country.

“Every leader has a unique relationship with their CEO,” Gass said. Schultz, she says, does his best to bring out the best in a diverse set of employees. As for her own reports, she sees mentorship as her “day job,” particularly with women. “We have many, many female employees throughout the company, and I view it as an honor to operate in a way that can inspire and engage.”

Brewer’s advice to those up-and-comers? Figure out how you want to lead. “I wish I had spent more time getting to know who I was, because it’s a challenge every day as a CEO,” she said. Confidence is paramount. “You have so many people that just want to march behind you that you can’t lightly what you say,” Brewer said. “Because if you give them partial marching orders it’s like a stampede.”

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