Magnet Wire Insulation
why there is not a constant ratio of the insulation area to the total
  area in all AWG conductors?


Because the important factor is the breakdown voltage of the insulation, and this is a function of thickness, not cross-sectional area.


  The second question is: can we take the advantage of the surface
  increase due to the insulation in thinner conductors and to design for
  more current capacity than thicker conductors?


No, because the typical application has the wire wound tightly against its neighbors. The ability of the overall winding to get rid of heat is a function of its exterior surface area, not the surface area of the individual wires.