Great lesson and demonstration! I think you see singers singing with a big open mouth in their middle register because they are trying to add more emphasis, when they are lyrical singers, singing dramatic repertoire. They are trying to create a bigger sound that they don’t have naturally. You see it very often with lyric baritones singing repertoire for dramatic baritones, or with baritones trying to sing bass repertoire. Or with lyric tenors singing dramatic tenor repertoire, and of course light sopranos trying to sing dramatic roles.
