Labor-intensive work such as taking a client's son or daughter on a college tour is often included in the fee, Nesvold says, and if those tasks multiply, profits go down quickly.

Bessemer had a profit margin of 13 percent in 2010 compared with 21 percent at Goldman Sachs Group Inc., whose internal family office is called Ayco Co. (Goldman chose not to participate in the ranking.)

Personal Service

Bessemer is owned by its employees and the descendants of Henry Phipps, a partner of Andrew Carnegie. Phipps started the firm in 1907 to manage his proceeds from the sale of Pittsburgh- based Carnegie Steel Co. Bessemer now serves more than 2,000 multigenerational families.

John Hilton, CEO of Bessemer, says the advantage of working for a private firm like his is that there are no shareholders to satisfy. And it means he can go the extra mile for his clients. One of his employees once waited in line at the Department of Motor Vehicles in New York for a client who had just moved to the city and needed a license. The Bessemer staffer called the client as she reached the front of the line.

No. 1 HSBC Private Wealth Solutions has grown partly by catering to Asia's newly rich. Last year, the region housed 3.3 million millionaires, according to Cap Gemini and Merrill Lynch, compared with Europe's 3.1 million.

Asia's New Billionaires

Bernard Rennell, global head of HSBC Private Wealth Solutions, who's based in Hong Kong, says his group targets families with $500 million or more. Asia's new billionaires often come seeking help with succession plans, Rennell says. HSBC came up with a bespoke solution for an Asian tycoon who wanted to pass along a controlling stake in his company to his seven children without giving all of them a chance to meddle in the company's day-to-day workings.

The banks' family offices have gotten bigger partly by buying boutiques. Bank of Montreal bought the name and client assets of MyCFO Inc., founded by technology entrepreneur Jim Clark, in 2002. Now called Harris MyCFO, it's No. 9 in the Bloomberg Markets ranking, with $18.3 billion. Atlantic Trust, No. 11 with $14.8 billion, is an amalgam of three wealth managers that Atlanta money-management company Invesco Ltd. put together from 2001 to 2004.

Kirk Bowman, a venture partner at Accel Partners, a venture-capital firm in Palo Alto, California, says he looked at the big investment banks before handing his fortune over to MyCFO.

"It's better to have someone who's investment-vehicle agnostic as opposed to someone who's got an unnatural incentive to put you in their product," Bowman, 45, says.