Since construction is mainly for patients who have had a mastectomy, so would a difference in size of each breast be considered reconstruction?

Yes: When reconstruction is done after a mastectomy, if the opposite breast requires additional procedure or reconstruction to achieve symmetry (to make it match the reconstructed side) it is considered part of the reconstruction. Generally it will be a medically covered service.

Depends: It would depend on your insurance company and the severeness of the deformity. Every woman has some differences between her breasts (give any plastic surgeon a ruler/marker/5 minutes and he/she will show you!). Where i practice this would likely be a battle but if you do have a significant lack of development of one breast or tubular breast, coverage may be attainable with good documentation.