As we know that molecules of a gas move far away from each other because they are held by weak Vander waal forces. So, when a balloon is filled by a gas then molecules of a gas strike at its walls leading to more number of collisions.
This will also lead to more expansion of the balloon. As we know that pressure is the force exerted n per unit area of a substance or object.
Thus, more is the kinetic energy of the molecules of the gas more will be the force exerted by them. Hence, more will be the pressure exerted.
