Question

why do small Bubbles have excess of pressure

Answer 1

Because of the surface tension of the liquid surrounding it. Any liquid surface has a force along it resulting simply from the fact that there is no liquid at one side of the interface and there is at the other. If the surface combs, tension is still there, and if it closes (a bubble), it continues to be. Then, for any bubble, inner pressure should equate outer pressure plus surface tension, so it is larger. Surface tension is not huge and can be neglected versus pressure for large systems. But the difference of pressure for a bubble is inversely proportional to the radius, so that the effect can be important for a small bubble

Answer 2

Because according to the Boyele's law the pressure inside a bubble is inversely proportional to the volume of the bubble.
So the smaller a bubble is the greater will be the pressure.

I hope it will help.