I'm a man in my mid-30s who has been working professionally singing pop, rock, and musical theater for most of my adult life. In the last few years, I've been training as an opera singer in the interest of mastering my instrument and working more as I age. There seems to be a consensus among my teachers and directors that my fach falls in the category of a heavier lyric tenor, although my latest teacher seems to think I could train as a baritone if I wanted. I've been reading a lot about vocal pedagogy for classical singing, and these teachers like to talk about music itself being "heavier" or "lighter" relative to each other. I sorta get the comparison; Wagner's tenor stuff seems to just sound better with bigger, louder voices, and be more forgiving of an uglier tone, although from a theory perspective I don't really know why. He also seems to favor a lower tessitura. By contrast, you clearly need a lot of agility to sing Ecco ridente in cielo, and I can easily see how a bigger, lower voice might not be able to handle that.

Is agility and tessitura all there is to it, or is there more? Are there aspects to the way music is written that contribute to the kinds of voices that should sing it? Can a huge Wagnerian tenor voice also handle Mozart? Enlighten me. =)