Matter exists in molecules. Solids have a great cohesive force and hence they take any shape. Liquids also have good cohesive force between molecules but not as much as solids. They take the shape of the container. Gases have very less cohesive force between molecles and hence they move randomly in the given volume. In the Macroscopic point of view, the volumes considerd are large compared to the size of the molecules. It is assumed that the volume under consideration will have enough number of molecules present in it, so that the definition of density, mass etc. will not alter eventhough there is movement of molecules in and out.
Let us take a volume comparable to the volume of molecule. Due to the random movement of the molecules, the volume under consideration may have molecules at an instant and may not have molecules at another instant. In that case, the definition of density and other properties will not have any meaning. Hence it is considered that, always, the volume under consideration will have many molecules so that the definition of the properties will have meaning.
