the average diameter of the ureter is 3-5 mm, and the lumen only opens when urine comes through, otherwise is collapsed. It is said that stones up to 5 mm may pass all the way. However, there are 3 difficult points to pass: UPJ, UVJ and the point where the ureter crosses the iliac vessels.
By the way, not only UPJ stenosis or genetic predisposition may cause stones. A normal person without any of these may pass a stone, and never again form another one.