You haven't grasped the meaning of dilation. Dilation doesn't mean that the bile ducts suddenly become larger. Magnesium Sulphate acts in the same way as the natural hormone CKC, which is a contraction of the gallbladder. At the same time that the gallbladder contracts the muscle that controls flow of bile tightens and consequently the pressure inside the biliary system increases and this causes the bile duct to expand; this expansion is called dilation.
The same dilation will occur when the oil and citric acid potion is consumed.