Volumes and Mass are the understanding of the internal size of the character. Think of the character as a balloon filled with air. The surface of the balloon contains the air inside. When you press on the balloon, there is still the same amount of air inside. The place where you press compresses and another part of the balloon will expand.
You need to think of your character the same way. They can stretch and squash or bend, but they can’t change their volume and mass, unless it’s for a reason dictated by the story.
