Originally Posted by don d.

Andy Walser was the innovator of the narrow bottom bracket design and narrower dropout spacing(by today's standards) that has tested out with improved aeroynamic advantages in wind tunnel testing. The narrow BB brings the knees closer together and provides a more aerodynamic profile. For some riders, it also increases power because of the repositioning of the legs in relation to the hip.

This design was copied by Trek in Lance's new TT bike that he abandoned because he couldn't adapt to the new position.

I haven't heard much about them weighing more or less, but they do provide aerodynamic advantages if you can ride them.

The narrow BB design from Walser's website: