If I could find my volumetric lung exerciser, I would give you a comparison of lung capacity with the chest up and chest not up. Part of this has to do with good posture. Most people tend to slouch and this is often reinforced by hutching over computers and game consoles. Lung capacity is increased by good posture. The chest up promotes resonance. Good posture includes keeping your chest up and this helps us produce a more resonant sound. I believe this explained more in the Adagio Fundamentals “The Body’s Concert Hall”.
Perhaps someone will chime in and explain this more fully.