A soap bubble has two liquid surfaces in contact with air, one inside the bubble and the other outside the bubble. Therefore the force due to surface tension = 2 x 2πrT
At equilibrium, P πr2 = 2 x 2πrT
So, P = 4T/r
Thus the excess of pressure inside a drop is inversely proportional to its radius, P ∞ 1/r. As P ∞ 1/r, the pressure needed to form a very small bubble is high. This explains why one needs to blow hard to start a balloon growing. Once the balloon has grown, less air pressure is needed to make it expand more.