If the MRI
is not showing any cause for obstruction then you need not worry. Postcholcytectomized state can also give rise to dilatation.
A 10mm duct becoming 8mm is suggestive of narrowing which can be extrinsic or intrinsic and needs to be evaluated. At times a Contrast enhaced Ct yields more information on such issues.
Get a stool occult blood examination done. If positive then a upper GI scopy and Ct may be indicated.