To be fair to OP, pros do typically ride a size or two smaller than what the average roadie would be on for a given set of body dimensions, and run much longer stems (120-140mm is normal). I think it's less about centering legs over the BB and more about getting lower on the front end, as a smaller frame size will almost always have a lower head tube.

It's also been discussed here in years past-

http://www.bikeforums.net/profession...ong-stems.html