In an interview, Christie said he agreed to the operation after turning 50 in September and he was thinking of his four children when he made the decision.
"I've struggled with this issue for 20 years," he said. "For me, this is about turning 50 and looking at my children and wanting to be there for them."
The Republican governor said the effort to lose weight wasn't motivated by a possible presidential bid in 2016.
"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 [his wife] and the kids and going, 'I have to do this for them, even if I don't give a crap about myself,'" he said.
Christie checked in to a surgery center Feb. 16 using a false name, a source told the Post.
The operation included placing a silicone tube around the top of his stomach to restrict the amount of food he can eat at one time.
"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.
Christie, seeking re-election to governor's office, never revealed his weight, but estimates have run from about 300 pounds to 350 pounds. Sources said he has shed nearly 40 pounds.
He hired the same laparoscopic and bariatric surgeon as Jets coach Rex Ryan -- Dr. George Fielding, head of New York University Medical Center's Weight Management Program. Ryan lost 100 pounds from 350 pounds after he had the procedure in 2010.
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