Credit: Tiffany Rose/WireImage

LeAnn's getting littler.

On Thursday, LeAnn Rimes hit the red carpet outside BOA in West Hollywood to lend her support for the Leeza Gibbons Dare 2 Care benefit.

With fiance Eddie Cibrian at her side, Rimes, 28, wore a short, form-fitting purple mini-dress and heels -- all emphasizing her surprisingly gaunt (though muscular) new physique.

"She looked unbelievably skinny," an onlooker says of the singer.

Although Rimes and Cibrian, 37, mostly kept to themselves during the fete, she sang two songs for guests (including her hit "How Do I Live") -- and gave a quick speech. (Other stars at the benefit included Jamie Lee Curtis, Jennifer Grey, Fran Drescher, and songwriter David Foster.)

"It's so important we must remember we are all human," said Rimes, whose extramarital affair with Cibrian (first revealed by Us Weekly) made headlines in 2009 and 2010. "We all make mistakes, and we shouldn't judge one another."