I think it is the size of the icon displayed. For example, if you have Graphic Converter on your system it shows a different folder graphic when Finder View is set to Icons than when set to List and different aggain when set to Columns. Do a Get Info on the Graphic Coverter folder and you will see two different icons.
Once upon a time - before Mac OS X - there was a wonderful application named Resedit that enabled users to look at resources in any type of file. It was possible to view all the different icon sizes that the file creator had included. Most times they are generic icons but sometimes, like Graphic Converter, they use different sizes of icons because they display better.