And the more operas I see, the more I think it's best to have a little weight onstage. I don't mean you should be unhealthily obese, but every time I go to the opera and the leading soprano is a skinny little thing, I feel like I lose her onstage. That's how I felt about Natalie Dessay in Manon. They had to dress her in bright clothes all the time, because you couldn't see her. You need presence!
That said, if you're not comfortable as a size 12, if you don't like the way you feel/look at that weight, by all means lose weight. But I personally think that people with more physical presence do better on opera stages.