'Biggest Loser' Star Sued By Fitness Company -- You Can't Get Fat Again!!!

As if re-gaining a bunch of weight wasn't bad enough, former "Biggest Loser" star Tara Costa is now being SUED for porking up again ... and it could cost her a lot of money, TMZ has learned.

FC Online Marketing -- which owns ILoveKickboxing.com -- has filed a lawsuit against Costa claiming it entered into a fitness promotional agreement with the reality star ... but she didn't hold up her end of the bargain.

The deal breaker ... FCOM says Costa -- who famously lost 155 POUNDS on "The Biggest Loser: Couples 2" back in 2009 -- had re-gained "far too much weight" in 2011 to represent the brand in photo shoots and public appearances.

FCOM says it paid Costa's foundation $45,000 in exchange for her image and likeness ... but since she got fat again, her image has lost all value to the company.

FCOM is now suing Costa for breaching the don't-get-fat agreement, demanding unspecified damages.

For what it's worth ... from Costa's recent Twitter pics, it appears she's back down to her fighting weight (or close to it) but FCOM insists the damage is already done. Attempts to reach Costa were not successful.