Bug - File size continues to grow

I think I've found a bug. I have a complex .obj file that I import, then I save this after applying durfaces etc. the object. Then I save this file as a .dn file. Then I create a new file, import the .dn file I just saved. The reason for doing this is that I have many variations I have to render, using the same object. I only want to add the object surfaces once. Problem is that sometimes I end up importing a previously imported object from a .dn file. This seems to cause the file size to grow! For simple objects, it's only a small amout, but for complex ones, it's significant!