Mike,
The reason that the inner bearing is usually larger is that it carries most of the vertical load of the spindle,i.e. the corner's weight plus any aero of gravitational down force (read "lead") and the outer bearing is there to carry the moment load of the wheel when it is turning and trying to twist off of the spindle. Making the both bearings the same size certainly is not a problem and many of the dirt track and circle track spindles are made that way.
Rex