A few weeks back, someone posted a link to a lecture about gallbladder surgery. The speaker was a surgeon. He said he has seen the common bile duct stretch to 20mm when there is a stuck stone and bile backs up. I'm sure someone will correct me if I'm wrong, but the 4cm stones I've seen have been significantly less than that in diameter, but 4cm long. I suspect some of these melt together in the intestines to form larger stones.
I have never heard that all Gallstones are hard, just the calcified ones.