Table 1 below shows the balance between gas volume and gas density. As the pressure increases with depth, the volume will decrease and the density will increase. This is what happens with any gas space in your body and equipment, for example, your ears, sinuses, mask, and lungs.
Table 2 below shows that even though the pressure increases, the lung volume remains the same. To do this, the amount of liters of gas that you inhale has to increase substantially due to the increased density of inhaled gas.
As absolute pressure decreases on a gas with ASCENT, the volume will increase proportionately. So, the volume of gas will expand. If a diver should hold his or her breath at 10 meters/33 feet and ascend with 8 liters of compressed gas inside the lungs that can only hold 4 liters (as in Table 1), the expanding gas would cause serious damage to the lungs.
Have you noticed anything? The biggest pressure and gas volume change is between 10 meters/33 feet and the surface. The volume of gas doubles on ascent or halves on descent. In only a couple of meters or a few feet, the pressure differential is large. Because of the rapid change in just 10 meters/33 feet, this is the area where you have to be the most cautious, especially with regard to barotraumas (which can occur when you hold your breath).