Pelvic area is definitely important. A friend who worked for Genex, and had several other cattle related jobs, including Co-op manager, (Now working in the Northeast for a feed company; has a degree in animal science, used to breed Maine Anjou) once worked for a large Angus ranch. He said they bred religiously for low BW without regards to other areas of calving ease until they were pulling 60 pound calves from cows with tiny pelvic areas.
BTW, for you Red Angus folks on the boards, this same guy worked for a time for David Ingalls, head of the Oasis Network of christian radio stations, and a pastor in Broken Arrow. Mr. Ingalls has a herd of Red Angus that my friend said were some of the best he'd seen.