Trouble With Gold-Filled

Shiny :-) vs. dull :-(

I recently had a request for some gold-filled rings. These are the coils I made from the wire I received, although they were made from two different lengths of wire. They were made at the same time, and tumbled together. The large coil is as expected - beautifully shiny. The small coils are dull and pitted on the surface.

Click to enlarge - you can really see the difference!

My supplier insists that both lengths of wire are gold-filled, so the question becomes one of mandrel size. Has anyone ever experienced gold-filled dulling down with smaller mandrel sizes? Or do you have any other explanation for the difference? 

Please ask your jewellery making friends as well!