Kevin Federline says it was hard seeing flabby photos of himself, especially the much-published shot of him playing golf last July in California.
"When I saw this picture, I did not think that I was that big," he tells Entertainment Tonight. "I thought it was Photoshopped, you know? And then I realized that I was just letting myself go."
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Adds the dancer-rapper, 31: "I looked pregnant. When I see pictures [of me] like that and people are calling me K- Fat, I had to do something about it. At my biggest, I was over 240."
He says he packed on all the pounds due to personal stresses, including a 2006 divorce from Britney Spears. "I went through a lot of things," he says. "I definitely say that those things contributed to me gaining weight for sure."
He says his children were one of the main reasons he signed on for VH1's Celebrity Fit Club: Boot Camp, which premieres Monday. He says he couldn't keep up with them during play dates. Adds the rapper, "I could barely make it up the hill on the side of my house."
Though he can't reveal how much weight he has lost, he says he is pleased with his new shape.
He also is thrilled he's on better terms with Spears, 28, with whom he has two boys, Sean Preston, 4, and Jayden James, 3.
"Me and Brit are good," he tells ET. "I don't talk about those things anymore. It's in the past. We've come a long way. and I'm happy where we are now. Everyone knows we had a tough road."