Porosity is a term used to describe the amount of void space between particles of a substance. The larger the particles, the larger the void space between them, the smaller the particles, the smaller the void space between them. What this means is porosity would increase with the use of larger particles; it would decrease with the use of smaller particles. Take pottery, for example; clay is used in the making of pottery because clay is an extremely small particle, with very little void space between the particles. This gives the clay pot excellent water retention properties. Sand, which is found in sandstone, on the other hand, is composed of large particles, which have lots of space between the particles. Sandstone is not an excellent choice for its water retention capabilities. If mixed particle sizes are used, you get a water retention factor somewhere between clay and sand, since you have a mixture of small, medium, and large particle sizes.