First off, I will define porosity here as the amount of a solid's volume that is occupied by empty space. As an example of empty space in solid objects and particles, think of the nooks and crannies in a sponge. The reason that the porosity of a particle is not affected by its "size" (total volume) is because in most applications, and especially in all types of engineering, porosity is described by something called the void fraction. In fact, often times scientists and engineers use the terms void fraction and porosity interchangeably. The void fraction (or porosity) of a particle is defined as the total volume of empty space within divided by the total volume of the particle. Void fraction is often represented by the Greek letter phi:
Thus, no matter how big or small a particle gets the porosity is always gauged by the ratio of empty space to total space occupied. This means that so long as the particle remains made of the same material with the same distribution of empty space, its porosity will not change regardless of total volume. In fact, this is how you should think of porosity: a physical property of a given material and the environmental conditions in which is exists that does not change with the system size (much like density). Properties like porosity and density are called intensive properties. The opposite of an intensive property is an extensive property, which does change with system size (e.g. volume, mass, etc.).