Soil retains water in the interstitial spaces between the soil particles; the amount of space in a soil sample is described as its porosity. Generally speaking, larger sized particles don't pack together well, so they tend to have bigger spaces between them which can pass a lot of water through quickly. Soil made from smaller particles will have less space between the particles, and so will be less porous.

A soil made up of mixed particle sizes has much less porosity because the particles tend to sort in a way that minimizes the interstitial space. Study this illustration and you'll see why:

Describe the relationship between particle size and water retention.  What would happen to the porosity if a sample of mixed  particle sizes were used ?

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.