Speaking at a news conference Tuesday in Newark, Christie said his weight will have nothing to do with whether he decides to run for president.
He says he never intended to make the decision public.
"I've struggled with this issue for 20 years," he told the newspaper. "For me, this is about turning 50 and looking at my children and wanting to be there for them."
Christie has never disclosed his weight, but it's been an issue throughout his political career. Christie said four years ago that then-Gov. Jon Corzine was bringing it up in a campaign commercial that accused Christie of "throwing his weight around" to get out of traffic tickets.
Comics including Jimmy Kimmel and David Letterman also have made fun of it. In interviews with Letterman, Orpah Winfrey, Barbara Walters and others, Christie has both joked about the issue and said solemnly that he's trying to shed pounds.
During a February appearance on "The Late Show with David Letterman," the governor pulled out a doughnut and said his girth was "fair game" for comedians.
Christie was soon angered by comments from a former White House physician who said she worried about him dying in office. The governor said Dr. Connie Mariano should "shut up."
Ten days after that, on Feb. 16, Christie had the surgery. He said the operation lasted 40 minutes and he was home the same afternoon.
Christie, who is in the midst of a re-election campaign, declined to say how much weight he has lost since the surgery.
"A week or two ago, I went to a steakhouse and ordered a steak and ate about a third of it and I was full," he told the Post.
The Republican governor is running for a second term in November, although his name is often mentioned as a possible presidential candidate.
"I know it sounds crazy to say that running for president is minor, but in the grand scheme of things, it was looking at Mary Pat and the kids and going, 'I have to do this for them, even if I don't give a crap about myself,'" he said.
The revelation about Christie's surgery came the same day that MSNBC host Mika Brzezinski's book featuring comments from Christie hit bookstores.
In the book, "Obsessed: America's Food Addiction - And My Own," Christie describes working out four times a week but not seeing major weight drops and hearing critics say that his weight shows he's undisciplined. He also talks about what others have said on Twitter, such as: "HEY GOVERNOR, WHAT DID YOU HAVE FOR BREAKFAST TODAY, ONE STICK OF BUTTER OR TWO?"
"For somebody like me who's had so much success in my life, and really been successful at everything I've tried, to not be able to be successful at this is incredibly discouraging," he said.
Approximately 160,000 stomach-reducing procedures of various types are performed each year. Gastric bypass, sometimes called stomach stapling, is the most common, where surgeons shrink the stomach's size and reroute food to the small intestine.
Gastric band surgery, best known by the brand name Lap-Band, is a less invasive and reversible alternative, where an adjustable ring is placed over the top of the stomach and tightened to restrict how much food can enter.
Candidates for gastric banding must have a body mass index of between 30 and 40 - plus a weight-related medical condition, such as diabetes or high blood pressure - or a BMI of 40 and higher. They also must have previously attempted to lose weight through diet and exercise.
Christie, who says he does not have any other significant health problems, has talked about working with a personal trainer since he first ran for governor four years ago.
"If you eat appropriately and chew your food, it works nicely," said Dr. Christina Li, a bariatric doctor at Sinai Hospital of Baltimore.
She said Christie has the resources to have people help him eat right and get exercise. While the band is removable, she said patients are told to adjust to having it for the rest of their lives.
Li said risks include infection, and that it does not work for all patients.
Dr. Jaime Ponce, who practices in Dalton, Ga., and is president of the American Society for Metabolic & Bariatric Surgery, said that people who have the procedure Christie had often lose 1 to 2 pounds per week.
Christie's procedure was performed by Dr. George Fielding, head of NYU Medical Center's Weight Management Program, who did the same procedure for New York Jets coach Rex Ryan three years ago.
The adjustable Lap-Band has been available in the U.S. since 2001 for the most obese patients. In 2011, the Food and Drug Administration expanded approval to somewhat less obese patients.
AP Medical Writer Lauran Neergaard in Washington and AP writer Katie Zezima in Newark contributed to this report.