when pressure decreases do the molecules shrink
This behavior relates to Charles Law of Gases (Gay-Lussac's Law also applies in terms of pressure changes in a rigid container). You may have noticed that car tires and balloons shrink in cold weather. This is due to the molecular motion of gas particles. The gas particles slow down as temperature decreases. This, in turn, decreases their force exerted on the container walls and surrounding particles. The reverse is also true if temperature is increased. Youtube has a 90 minute TV special that relates temperature and absolute zero to the behavior of matter.