2011 is turning out to be a very strong year for health insurer UnitedHealth Group ( UNH) -- shares of the $53 billion firm are up more than 36% since the first trading day of January. Of course, management has been contributing to investor returns as well, increasing its payout by 30% recently. The sheer size of UnitedHealth's operations give it a massive advantage over its peers. In total, the company counts more than 77 million Americans as its customers, a network size that gives the company major negotiating leverage when determining costs on both ends of the spectrum -- medical providers and insurance customers. While that's traditionally helped drive solid margins for the company, the major question remains the constantly changing fate of healthcare reform legislation. There's little question that many parts of its current incarnation will be costly for UNH -- but changes could derail current estimates. Still, the company has shown in the past that it's able to adapt to regulatory changes and remain a high performer. While a relatively low dividend yield makes this a less viable core holding for an income investor, the high profile of this health insurance giant makes its dividend changes significant nonetheless. UnitedHealth, one of the top holdings of Bill Miller's Legg Mason Capital Management, shows up on a recent list of 6 Stocks on Pace to Double This Year.