Never hold your breath while scuba diving!

The lung capacity on an average adult is 6 Lts of air. This air volume will be affected by the increasing underwater pressure as the diver descends, as stated in this article.
As the diver descends, the lungs volume will decrease as a result of the increase of pressure. This change of volume will be proportional to the number of atmospheres at a given depth.
At 10 Mts / 33 Feet, where the pressure is 2 bar /ata, lung volume will be one half smaller than at the surface. At 30 Mts / 100 Feet (The recommended depth for recreational divers) the lungs volume will be as small as one quarter smaller than at the surface.

Pressure in our lungs

But even when the lung volume keeps becoming smaller as the diver descends, the air capacity remains the same. So when we breath compressed air at depth, we are actually adding an extra amount of air to our lungs in order to equalize them to the surrounding pressure. If a diver holds his / her breath while scuba diving, serious lung damage may occur.
That is why rule number one of scuba diving states: "Never Hold your Breath"