Shea Walsh / Invision/AP 1 / 7
Shea Walsh / Invision/AP 1 / 7

When Kevin Federline's waistline ballooned last year, he was hit with all sorts of not-so-nice nicknames: K-Fat, K-Well-Fed, K-Overfed, Kevin Fatter-line.

You get the idea.

Now that the former Mr. Britney Spears has slimmed down after a stint on VH1's "Celebrity Fit Club: Boot Camp," he's opening up about his weight gain, which he believes was caused by depression, inactivity and overeating.

"Once you get depressed, you don't really feel like doing anything," the former backup dancer-turned-former rapper-turned-well-upholstered-full-time dad explains to "Access Hollywood." "The more that you gain, the less that you want to go out. You just start getting discouraged about yourself, and for me, that caused me to eat more and it was just a bad downward spiral."

Federline says he realized he'd become "way, way overweight" after paparazzi snapped him shirtless in a Miami pool.

"I stopped dancing. I hadn't danced for five years," admits the father of four. "Between not working out and overeating and sittin' on the couch all day ... it was just a combination of all those things and I wound up gaining weight."

RELATED: A field guide to celebrity diets

He cites his divorce from Britney, during which he gained custody of sons Sean Preston, 4, and Jayden James, 3, after her public meltdown, as a contributing factor to the whole letting-himself-go process.

"Everybody knows what happened with me and Brit," opines Kevin, 31. "I'm not going to say that's the total cause of it but, I mean, just not being happy with myself was probably the main part of my depression."

These days, things are "getting better and better and better" between the exes, enthuses Federline, who is still coupled up with statuesque blond volleyball player Victoria Prince. "I mean, I'm really happy the way that my life is going right now, I really am. I think, you know, I have come a long way. All of us have … my whole family. It's gotten so much better from where we were."

As for Britney, who was spotted earlier this week taking Sean and Jayden to a movie with sometime squeeze Jason Trawick, "I'm really happy for her," says K-Fed of his still-conservatorship-controlled ex. "I'm very happy for her."

And for those suspicious types who continue to wonder whether Federline bulked up simply to land a paying gig on "Fit Club," he insists that wasn't how it happened.

"They called me and they asked me if I wanted to do it and I had maybe 24 hours to decide because the show started taping the next day after that weekend," recalls Kevin. "It just couldn't have come at a better time."

Read more Hot Gossip