By the by, a quick sweep over to the MD-80 information section of airliners.net shows that the MD-80 has a range of 2920 nautical miles fully loaded, and WebFlyer indicates that the distance between BOS and SFO is about 2700 nautical miles. If it was one of the MD-87s, it has a range of almost 3,000 NMs.
Both could make the eastbound journey without any weight restriction at all and without the aid of tailwinds -- and this plane was far from full.