Oulu University; Review of the Literature Imaging of biliary carcinoma, fistula and primary sclerosing Cholangitis and percutaneous metallic stenting in malignant biliary obstruction Chapter 2; 2.1. Anatomy and function of the biliary tract
The common bile duct varies from 5 to 17 cm in length, and its diameter varies from 0.3 to 1.5 cm and may become markedly distended by biliary obstruction.
The fact that these ducts can become markedly distended by biliary obstruction indicates some expansive, elastic capabilities to the bile duct. When the bile duct is relaxed with magnesium, the pressure from a lodged stone and strong gallbladder contractions (from heavy oil ingestion) will cause a stone to expand the duct along it's length as they slide out.
Also as lisamarie posted below, stents can be inserted in the bile duct that have a maximum diameter of 12 mm. Clearly there is some expansive capability here that is not being reflected in the standard reference diameter number.