Question

why the gas bubble at the bottom of water increases in size as it rises above the surface ?

Answer 1

The pressure at the bottom of water is higher than the pressure at the top. 
When the bubble is at the bottom, the pressure acting on it is higher than when it is at the top. So at the bottom, the size is smaller due to higher pressure and the size increases gradually as the pressure decreases when it starts rising.

Answer 2

When the pressure is more, the volume is less.
At bottom of water, pressure is higher due to weight of water column above it. So volume of bubble is less. When it goes up, the pressure reduces gradually and volume increases.