Originally Posted by man of calibre
So it stands to reason that if headsets are improved by putting the bearings inside, bottom brackets should be pretty close to the optimum design because the bearings are already internal. Wrong! We are now supposed to accept as given that putting the bearings on the outside will make a better design.
Uhhh... no. BBs and headsets withstand very different sets of forces. Headset force is mostly compressive; BB force is mostly sheering.
There are several reasons for external bearings. In sheer the bearings form the fulcrums in a class three lever. The farther apart the fulcrums (assuming equal lever arm length), the less the load on each. In addition, the farther apart the bearings the less any play in the bearings appears at the pedal. And if that's not enough, external bearings aren't restricted by the diameter of the hanger tube. Larger bearing allows larger axle which can be stronger and roll easier, and longer races wear better.
Btw, if you think that headsets are standardized, think again.
Headset "standards".