Wagner didn’t invent the giant orchestra. I remember looking down into the pit at the Wiener Staatsoper during a performance of Weber’s Der Freischütz (1821) [that’s the opera in 1821 and not my viewing of it] and seeing about 20 French horns. This was probably bogus and would only have been used in the overture and hunting chorus. If memory serves, Berlioz produced pieces with giant orchestras, the earliest being the Grande Messe des morts of 1837. It is amusing to read that Berlioz didn’t like Wagner’s music.
Richard Wagner composed as possibly no other person. He liked a dense, roiling soup of sound that best resembled an endless development section of a symphony. He liked it to sound thick, an effect that is most easily achieved by adding a lot of sound in the middle of the chord. He even invented an instrument, the Wagner tuba, basically a low French horn, that enhanced this low groan that he preferred.
Above and in the midst of this thick groan of sound singers are expected to sing. Where Verdi intuitively thins the sound to feature the voice his orchestra is accompanying, Wagner pays no heed to the singers’ difficulties and does nothing to help. He cannot have been unaware of the problem, because when the Festspielhaus at Bayreuth was constructed, he arranged to have the orchestra virtually covered with a kind of lid. He knew what a problem it was to sing his music, and he didn’t care.
It helps with Wagner to have a bit of edge in the voice. A big loud voice with an edge manages best. Some try to get by with just big, but they are at a disadvantage. The low groan in the orchestra makes it necessary to have a heavy voice to balance it. The tessitura of Wagner roles is not as high as it is for Verdi and the Italians. A Wagner soprano might well develop from a pushed up mezzo like Waltraud Meier, or Janis Martin. To sing over the Wagner orchestra, the voice needs to be big, loud and have a knife like edge to it. There are technical implications to this, but if you are trying to do it on technique alone, you are going to be in trouble. In fact I would have to say that you either have the voice or you don’t, and should do the best you can to float above the orchestra instead of pushing through it. Even bigger generalization: sing up to Wagner, never down.
There is a story circulating on the blogs that Solti once asked Renée Fleming to sing Isolde. She wisely refused. What could he have been thinking? But try to remember that when Kirsten Flagstad came to him to be coached as a Wagnerian, she had been a soubrette. We’re all glad that Renée turned him down.