A big Sphere Of Influence is going to attract lots of people because it will have lots of facilities.
However, the smaller the sphere the less facilities, which means people from smaller towns (smaller spheres)
will want to go to the large towns (big spheres) for shopping/entertainment. People are usually willing to travel a long
distance to go shopping for comparison goods (luxurys). People are less likely to visit a small town (small sphere) due to the
fact there is not much of interest and only has convenience goods.
In this diagram the comparison (cinema's, restaurants, shopping center..etc) http://uk.answers.yahoo.com/question/index?qid=20100206022139AAxPAHG goods are in the orange sphere and possibly the blue sphere. Convenience goods (milk, toilet bread, flour..etc) are in the green sphere and the blue sphere.