By Gary Strauss / USA TODAY

Super Size Me is a super-sized hit among documentary films.

The quirky movie chronicling the fallout from a 30-day, McDonald's-only diet has quietly pulled in $6.2 million since its release May 7.

Super's four-week take is hors d'oeuvre-sized compared with the $313.6 million gobbled up by early summer blockbusters like Shrek 2. Yet as Hollywood mega-films open big and fizzle quickly, Super Size Me is gaining traction. It's already the fifth-best-performing documentary ever.

"In the face of super-size competition, it's doing phenomenally," with three weeks in the top 10, says Paul Dergarabedian of tracker Exhibitor Relations.

Super Size Me gained critical mass at the Sundance Film Festival, where Morgan Spurlock won the top documentary director award. Spurlock, Super's human guinea pig, credits the film's success to its humor, rare for documentaries. "We present things in a way that's not preachy or chastising," says Spurlock, who is writing a book on the fast-food industry.

Gary Goldman, president of the company that runs the Esquire and Mariemont theaters, says he is "very satisfied" with how the film has done locally. "It's not the highest grossing documentary we've had, but it is certainly among the best," Goldman says. Also stoking interest:

• Timing. Up against few rival documentaries, independent films or big comedies, Super Size Me is mushrooming beyond an art-house favorite.

• Concept. Super's essentially one-note premise is smart in its simplicity. "Being able to describe what it's about in one sentence, everyone can relate," Dergarabedian says.

• Consumption. "Everyone eats fast food, so people relate," says Matt Cowal of E Street Cinema in Washington, D.C.

• Reality TV. Super appeals to those fed a steady stream of reality TV, especially young viewers. Says eighth-grader Ruben Anders of North Hollywood, Calif.: "It was funny, informative and brilliant."

• Marketing. Backed by an irreverent ad campaign, Super was quickly rolled out beyond New York and Los Angeles. "We didn't wait for critics there to champion the film because the entertainment factor was huge," says Michael Silberman of distributor IDP.

Super, which will expand to 250 from 205 markets, could pull in $10 million, says Brandon Gray of BoxOfficeMojo.com.

Made for $65,000 and marketed on a shoestring budget, it's among the most lucrative documentaries ever.

"It's a unique alternative to a big blockbuster," says Gitesh Pandya of BoxOfficeGuru.com.