A svelter belter: Soprano Deborah Voigt has slimmed down but that fabulous voice is as big as ever

Four years ago, Deborah Voigt hit the headlines by not singing in Ariadne auf Naxos at Covent Garden. The Royal Opera House dropped her from the cast, the reason being that Voigt, possessor of a fine dramatic soprano voice, was too large for the little black dress the production demanded she wear.

The American soprano has since undergone gastric bypass surgery and shed more than 100 pounds. Now, transformed, she is back at the ROH to take up the same role.

Voigt, 47, insists that the Ariadne incident made little difference: she had struggled with her weight for years and knew that she had to take action for the sake of her health. Wryly, she remarks that at least the little black dress had the effect of "getting my name out there".

The risk in such weight loss is that when the bulk goes, the voice it supports goes with it. When Maria Callas slimmed down, many felt it was to the detriment of her voice. But Voigt's, a gigantic, glowing scimitar of a soprano, has come through sounding as fabulous as ever.

Voigt says, though, that producing it is no longer as effortless. "I have to think about the process of singing a bit more," she says. "Because I had so much extra weight, when I took a big breath, that weight would automatically engage my abdominal muscles and the sound would just come flying out. That's not the case any more."

There's been a psychological effect, too. "You're not the same person. You feel different and your emotional life is different. I still tend to think of myself as an extremely large woman, and that will probably persist." But she's certainly happier on stage. "I can play dramatic parts with more conviction and more believability," she says. "I sang my first performances of Strauss's Die Aegyptische Helena here in London a few years back, and the text goes on and on about how she's the most beautiful woman in the world. But when you're walking around with all those extra pounds, there's no way you can believe that about yourself – at least, I wasn't able to."

Voigt began to sing as a child in church in Bible belt America, taking part in musical theatre at school and gospel music on Sundays – but it was sheer chance that determined her direction. "It was by singing in church with my mother that I realised I could connect with an audience this way," she says. "At high school, I started taking voice lessons with our choral conductor's wife, and she happened to be an opera singer. If she'd been a gospel or pop singer, I might well have gone down a different path."

Her teacher suggested Voigt find an aria she liked. Voigt didn't know what an aria was. "I found something interesting and took it to my teacher. She got this funny look and said, 'You know, this is usually sung by a tenor.' It was 'Nessun dorma'! She suggested we look at Cherubino instead..."

Now, her favourite roles are the peaks of the dramatic soprano repertoire, characters with strength and heart: Strauss's Salome, Wagner's Isolde. And the lead in Ariadne auf Naxos has been crucial, too: "I've made my debuts in several houses in this role, and it was the first one I sang that really attracted press attention and support," she says. "The opera has everything – comedy, tragedy, great love story and absolutely gorgeous music. This production is very different from the others I've done. It's more modern – clearly, with the little black dress! It's a great cast, and the conductor Mark Elder is working our tails off."

Her first Salome, complete with stripdown in the "Dance of the Seven Veils", was at the Lyric Opera of Chicago in 2006. "I always knew it would be a good role for me, but I never dreamed I'd have a chance to do it on stage." Apparently, Voigt used to joke that in her case it would be the "Dance of the 77 Veils".

It's been a long journey to Covent Garden, and there are challenges ahead. Voigt is preparing for her first Brünnhilde, scheduled for the Metropolitan Opera in New York in a new Ring production by Robert Lepage. She's taking on Strauss's Elektra, and singing more recitals. And she's dreaming of the songs of Broadway. "A show of Broadway music would excite me more than anything," she says. "I did my first evening of this type several years ago for Classical Action, raising money for Aids research and support. We did a show called Deborah Voigt on Broadway. Being American, this is the music I grew up with."

Voigt is setting up a foundation linked to the opera company in her home town, Vero Beach in Florida. "The idea is to help young singers bridge the gap between graduating and establishing themselves in the profession. It's a mentoring programme: each year I'll choose a young singer to come on the road with me for six weeks, to whichever opera house is the most welcoming, and they'll have lessons with me and the staff there."

It's been some years since Voigt last sang at Covent Garden, so her return is eagerly anticipated by fans who'll be seeing and hearing her for the first time since her operation. And even if Voigt's shape is svelter, she is still thinking big.

'Ariadne auf Naxos', Royal Opera House, London WC2 (020-7304 4000), in rep from 16 June to 1 July