Can't understand when air bubbles begin in deep water come up and those bubbles become large.

When air bubbles begin in deep water come up and those bubbles become large.

Have you done pressure in liquids yet?

The pressure in a liquid is given by the formula:

P = h\rho g

where P is the pressure,
h is the height of liquid above the point in question
this symbol is called rho, and denotes the density of the liquid
g is the acceleration due to gravity.

You will see that the deeper (the larger the height of liquid above a point) you go in the liquid, the greater the pressure. This is why when you try to dive deep, you can feel being pressed harder by water.

Well, a bubble of gas experiences the same effect. Deep down in the water, it feels the pressure of the water.

As it rises through the liquid, the pressure decreases as the height of water above it decreases. Since it's less compressed, it can acquire it's 'normal' size.

I hope this helped! :)