The country star, who hit the milestone on Nov. 10, overhauled her diet and worked with a trainer to drop a dress size.
"People think I lost more, but I'm short," she recently told PEOPLE backstage at the CMA Awards. "I'm only 5'4" so when I gain, I gain everywhere, and when I lose, I lose everywhere."
So how did she achieve her fitter frame? The old fashioned-way: portion size and exercise.
The singer drinks store-bought prepared pressed juice in the morning ("I don't do the juicer myself because I hate to clean it. It's a mess and I'm not that disciplined!" she said, laughing) and watches her portions. "I haven't given up everything!" she confessed. "I try to just cut everything in half."
As for her workouts, Lambert brings her trainer on tour with her, but they keep it basic. "We do cardio, a lot of circuit training. We do things with my body weight, like lunges," Lambert said. "A lot of stuff with [resistance] bands. The stuff nobody loves, but it works!"
Her husband, Blake Shelton, 37, is all on board for her makeover, even though he was happy with her just as she was.
"He said, 'I have never had a problem with any way you look at any point – you are beautiful to me at any size and every size,' " she explained. "But he has been proud of my hard work on this. He brags on me. That makes me feel great."
Lambert and Shelton celebrated her 30th birthday at home, just days after the two were named male and female vocalist of the year at the CMAs.
"We did have fun," Shelton told PEOPLE Tuesday night following The Voice's Top 12 elimination show. "She's 30 years old now, and it's a lot of fun to be able to poke fun at her for finally being in her 30s."
Added Shelton: "She even said … 'I can't use that I'm-only-in-my-20s excuse anymore about anything. Now it's time to be responsible.' "
Still, he couldn't resist giving his wife a playful jab.
"We'll see if she lives up to that," he said with a laugh.
Read more about Lambert's weight-loss secrets (including more details on her daily diet and her favorite cheat treat!) in this week's PEOPLE magazine, on newsstands Friday.
Reporting by MELODY CHIU