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Kevin Federline says he was as flabbergasted as everyone else by those infamous spring 2009 "K-Fat" photos, in which he weighed around 240 pounds.

"When I saw this picture, I did not think I was this big," Federline, 31, tells Entertainment Tonight of his pudgy look on a golf course. "I thought it was Photoshopped, you know? And then I realized that I was just letting myself go."

The ex-husband to Britney Spears and dad to her two boys knows why he "let go," too: "I went through a lot of things and I definitely say that those things contributed to my weight gaining for sure," he says of his high-profile divorce and custody battle with Spears, 28.

When he was at his heaviest, Federline says "I could barely make it up the hill on the side of my house." Slimmed-down again, Federline won't say how much weight he's lost -- that will be revealed on VH1's Celebrity Fit Club, which premieres next Monday.

His growing sons Sean, 5, and Jayden, 4, help him burn calories. "It was easier to eat more when they were younger because they weren't as active," he explains. "Now all of them want to play sports, hit the T-ball and run around the backyard and ride bikes."

As for his famous ex-wife, he and Spears are getting along these days, he says. "Me and Brit are good. It's … you know ... everybody knows we've had a tough road but I'm happy where we are at."