"She looks amazing," Blake Shelton told PEOPLE at rehearsals on Monday of his fiancée, who hired a personal trainer to travel with her on her most recent tour. "She set a goal for herself, worked hard and I am so proud of her."
For her part, Lambert, 26, is doing more than just watching what she eats. "My trainer's been kicking my butt," she said at rehearsals. "It's not so much losing weight as toning up, getting rid of the jiggles and wiggles."
Continues Lambert: "I'm down to a size 6. I had to have all my clothes taken in – so I'm really excited about that."
Lambert will show off her fit figure during two performances – a John Prine song from her album of the year nominated Revolution and, again, with Sheryl Crow in a tribute to Loretta Lynn – at Wednesday's awards show, which airs live on ABC (8 p.m. ET) from Nashville's Bridgestone Arena.
As for her astounding nine nominations, Lambert knows she can't win them all.
"I'm competing against myself in three categories," she says. "I hope I don't knock myself out."
Either way, the night will end with a big bang – and a large pizza. "I'll be 27 on Wednesday," she says. "After the show, I'm going to get together with my family and friends and celebrate my birthday."
Continues the singer: "I plan to eat! I haven't eaten pizza in 30 days and I can't wait!"