As Gary said, you can only go as narrow as the attaching pins. I just had a spare 16" narrowed to 10", the shop quoted me $150 but only charged $110 at the end. Although I would have liked narrower, I have a thumb which is 9" so that limited me. If you have a lot of clay or wet soil and trenching to do you can cut some (most) of the sides out of the bucket which will remove most of the suction. Just don't use the same bucket for heavy, rough digging as much of the strength is lost also.
Search for 'Clay bucket' Google images will get you some ideas.