Kevin Federline appears to have put on some serious weight, showing off a [cough] somewhat fuller figure while golfing with a friend in L.A. on Tuesday.

His two-time baby mama Shar Jackson explained that the 31-year-old former aspiring rapper (who wasn't exactly thin prior to this) had put on "daddy weight! When you're a full time parent, sometimes you can't focus on you."

He can focus on smokin' weed, sitting on his ass and ordering wings, apparently. Victoria Prince needs to get this guy into some other sports besides golf ...

Fat as he is, K-Fed's romance with Prince, a former pro volleyball player, seems to be heating up. The two took his sons Sean Preston, 3, and Jayden James, 2, out for a fun dinner at a Japanese restaurant in Fresno, Calif., over the weekend.

A source close to Federline says Prince is "so good" with the boys, who may or may not be pulling some covert ops and spying on her for their mom Britney Spears.

The couple - who bowl together on a team named Party Animals - "started off as friends, but it's become more," a K-Fed pal added. "It's been getting serious."

Unlike Britney Spears, laid-back Victoria Prince "brings sanity and normalcy to his life," a friend adds. "He's a lot less closed-off. Everybody is thankful he has her."