But I don't understand this justification as it's just opera, and grand librettos are just presumptuous,
and at the same time demand singers to look the part.
I'm done with this.
The first act of Tristan is supposed to take place on a boat but last time I saw a staged production it was inside of an opera house, not on a boat. The second act was supposed to be in Cornwall but it was actually in London, I could tell because Covent Garden is in London, not Cornwall.
I recently saw the Ring here in San Francisco and in Das Rheingold Alberich was supposed to turn into a giant snake beast, but the singer just went behind a wall so we couldn't really see him most of the time. He didn't actually shapeshift. Same deal for when he was supposed to turn into a toad.
But while it can be interesting to see how various elements of an opera are represented on stage that's really more a meta-concern than a part of the opera (in almost all cases). We have to work with the creators of the opera to accept that we are not seeing reality on stage. Some reality has to intrude to be able to present an opera on stage. Sometimes that distance is useful/used by the composer and/or director/crew; often it is not. We can examine and analyze that, but it is often best to focus on elements that work with what the opera is doing. And I consider the bodies of the singers to be one of those realities that is a necessary part of putting opera on stage.
Also I don't think there's anything in Tristan und Isolde
that gives the ages of the characters other than the "young" sailor that has the initial song that Isolde gets mad at. There's also nothing that says anything about the weight of the characters. Older people fall in love. Overweight people fall in love. Whatever singers we're presented with... people like them have fallen in love, magic potion or no.
One can be a gross creep and insist that they must be teenagers or be a cruel fat-shamer and assume that they can't be above average weight or whatever, but none of that has anything to do with what was written.