Why are your lungs more like sponges or balloons?

More like balloons: Lungs maintain their expansion by a tight seal to the chest wall. Like a balloon, if that seal is broken, the lung will collapse inwards. Sponges have a lot of cavities, but will maintain their shape whether full of water or air, or not. You can squeeze a sponge to a small shape but it will quickly expand to its usual shape.