It’s a truism that singers’ voices grow bigger with age. Like all truisms, it’s a dangerous idea to live by. Singers these days are almost expected to rise through the vocal ranks, singing ever larger roles until they implode on Verdi or Wagner, as if everyone were able to sing the biggest music in the repertory by the end of his or her career.

The soprano Sondra Radvanovsky, however, is getting smaller.

“Many people have said, Sondra, why are you taking a step backwards vocally?” she said.

Admittedly, Radvanovsky’s powerful soprano is far from small. But the singer is bucking what passes for conventional wisdom these days. Having sung the major Italian roles of the so-called spinto repertory — the big meaty parts like Verdi’s Aida or Puccini’s Tosca — she’s now moving back, chronologically and in terms of vocal heft, to earlier music. Having already appeared at the Washington National Opera as Donizetti’s Lucrezia Borgia in 2008, this week she is opening the season with the same composer’s “Anna Bolena,” a bel canto showpiece, written in 1830, about the doomed queen of England better known to English speakers as Anne Boleyn.

“The only difference for me between the Donizetti and the Verdi is [that Donizetti is] just a little more filigree, the singing,” said the 43-year-old soprano last week, sitting in her hotel room in downtown Washington before an evening rehearsal. “The problem is finding a balance between the vocalism and the acting. Because this music is so delicate. . . . Especially for a larger voice like mine, the temptation is just to go with the temperament and give it full (volume), and I have to really watch that.”

Sondra Radavanovsky as Anna Bolena. (Cade Martin)

Bigger isn’t always better. It’s a hard lesson for opera singers to learn. When you’ve spent your career training to make yourself heard in an auditorium of 3,000 people without a microphone, you can come to believe that the point of the exercise is to make as much noise as possible. As a result, too many voices have foundered on the shoals of roles too large for them. Perhaps because of the emphasis on size, there are more singers around today who can do Wagner than there are genuine Verdians. So that Radvanovsky has emerged as a genuine Verdi star is hardly a surprise; she’s a fine singer, and she has virtually no competition.

It certainly wasn’t an overnight success. Audiences, and administrators, seemed particularly slow to warm to Radvanovsky. “I think my voice perplexes people,” she said, calling it “unconventional.” Others have called it “cool” and “reserved.” Born in Illinois, she resolved to become an opera singer when she saw Placido Domingo singing in “Tosca” on television when she was 11; but although she was taken into the Metropolitan Opera’s prestigious Lindemann Young Artist Development Program at 25, and has performed pretty steadily with the company ever since, she never seemed to be a particular favorite.

As recently as 2009, she had no upcoming Met contracts after the new production of “Il Trovatore.” That production, however, was a turning point: After it, the Met embraced her, and she now has major contracts with the house through 2017. The highlight, over the next several seasons, are the three Donizetti operas about English history collectively dubbed “the Three Queens” — “Maria Stuarda,” “Roberto Devereux” and “Anna Bolena,” which she’s doing in Washington first.

Not many singers have taken on all three; indeed, Peter Gelb, the Met’s general manager, originally conceived the trilogy for superstar soprano Anna Netrebko, who opened the Met season last year with “Anna Bolena,” but wisely decided that the whole trilogy wasn’t for her.

It’s not unusual for a large-voiced singer to labor for years before coming into the spotlight. But Radvanovsky had additional obstacles. In 2002, shortly after her husband had quit his job to devote himself to her career full-time, she had surgery to remove a node on her vocal cord — a result not of improper technique, but of a sloppy intubation performed on her when she contracted pneumonia as an infant. Her doctor, Steven Zeitels, last year performed a similar procedure on the singer Adele.

After the surgery, Radvanovsky essentially had to relearn how to sing — but a new lightness and ease quickly made itself felt. When a singer has a growth on a vocal cord, she has to work harder to get the cords to close cleanly; and Radvanovsky had been dealing with this particular impediment all her life. Once the node was gone, she says, “I really had to learn to sing 50 percent less.”

Her voice certainly sounds comfortable in the big Italian roles like Tosca or Aida. But it was Lucrezia Borgia at the Washington National Opera in 2009, her first bel canto role, that she says caused a light to go on over her head. It wasn’t only the part; Lucrezia was the first role she had prepared for exclusively with her coach of 17 years, Anthony Manoli, after parting ways with her longtime voice teacher, Ruth Falcon, and she felt that some long-standing issues with pitch and technique had resolved as a result.

“Only now,” she says, “I’ve found what my voice really wants to do. The heavier, dramatic bel canto stuff. Yes, I can still do all of the spinto repertoire, but I really feel the Lucrezia Borgia is my true voice.”

Only today would bel canto be considered a step backward. The elusive term bel canto is now often associated with smaller voices, but the hallmarks of the style are not vocal size, but flexibility, accuracy, and range. Indeed, in turning to this repertory Radvanovsky is embracing tradition: the two leaders of the late 20th-century bel canto revival, Joan Sutherland and Maria Callas, both had impressively sized voices, and Callas moved back and forth between spinto and bel canto roles very much as Radvanovsky does.

The Three Queens are also a plum for a singing actress. Although they are fictionalized versions of history, they are based enough on fact that Stephen Lawless, who is directing his Dallas production of “Anna Bolena” at WNO, compares them to Shakespeare’s history plays.

“I think Donizetti gets a little bit of a bad rap,” Lawless says, opining that people compare his comedies unfavorably with Rossini, and his serious work with Verdi. “Actually, he’s a great dramatist.” He adds, “If you look at the comedies, there are always dark serious moments; and equally in the serious operas there are always moments of dark, dark humor . . . . I think that’s the mark of a master dramatist.”

Radvanovsky has certainly prepared for the role with old-school seriousness and focus, carving out time in her schedule to learn it. That time is inviolate. Last spring, the Met needed a substitute singer for the broadcast of “Aida;” Radvanovsky had just learned the role, and she was going to be in New York anyway, working on “Anna Bolena” with Manoli. Most singers jump at the chance of a Met broadcast. Not Radvanovsky.

“I said, I’m sorry, Peter, I just can’t,” she now says. “I have to work on this Bolena, especially since I’m doing all three at the Met. You want me to learn it right the first time.”

Preparation isn’t only about the musical side, either. “She works like an actress,” Lawless says admiringly.

“Nowadays people pay so much money [for tickets],” Radvanovsky says, “and it’s such a visual society we live in. If people want to just listen to pretty singing, they’ll put on a recording of Maria Callas, or Joan Sutherland, or [Montserrat] Caballe. But they want the whole package nowadays. And with the Three Queens, you have every ability to give it to them.”

Anna Bolena

continues at the Washington National Opera through Oct. 6.