The implants may drop, but not push out to the side over time.

It sounds as if you would like a big size implant for your frame.  With your narrow chest, a high profile implant may be your best choice.  I have become less enthusiastic about the tear drop implants.  I used them for many years and have seen rippling and deflations at a much higher rate than the smooth wall saline implants. 

The only way an implant can migrate laterally, or to the outside part of your chest, is if your surgeon over dissects the pocket.  The implants do have a tendency to drop or droop over time, but this is normal.  I would discuss your final size wishes with your doctor, not just the size of the implant.  When done properly, high profile implants have a very natural look.