https://youtu.be/UpOXecWC5Dw This video shows changes in the shape of the vocal folds as they switch between head voice and chest voice. Since we are viewing the vocal folds from the top downward we can see the folds elongate for head voice. But for a more complete understanding we would need a side view to see that the vibrating surface also becomes thinner in the head voice. https://youtu.be/-f9YPVYz1_I This video shows the vibration of the vocal folds as the singer begins in the whistle and descends very quickly into the chest and vocal fry. Notice that in the whistle, the vocal folds are stretched long and thin. Only a tiny segment of the vocal folds is vibrating. You can see this as the small opening of the glottis (space between the vocal folds) that appears on the bottom of our screen. As the voice descends into chest voice, the vocal folds shorten, and the entire length of the folds vibrates.