Should Implants Be Placed Under or over the Muscle for a Slim Frame?

It's a great question. I strongly prefer under the muscle implants because the muscle provides another layer of camouflage for your implants. The muscle contours the top of the implant to give it a more natural slope and hide rippling of the implant. Implants under the muscle have a decreased risk of scar tissue forming around the implant (capsular contracture).  Also, breast cancer screening with mammography gives better images of breast tissue when implants are placed under the muscle. Although an implant on top of the muscle can be a good choice for some women, for very thin women there is not enough camouflage for the implant to look natural. When considering breast augmentation make sure you find a board certified plastic surgeon who will take the time to answer all of your questions and explain all of your options, so that you can decide if breast augmentation is right for you.

If there is thin skin and soft tissue, it is best to place implants in the submuscular plane. The more soft tissue between the implant and the outside world, the better the result. There will be less visibility of the implant edges, less chance of rippling, and a more aesthetic contour i the upper portion of the breast. If the nipple position is above or slightly below the fold of the breast this is the best approach.

Hard to say with out examining you but it sounds like you should go under the muscle.  Better coverage of the implant, less chance of encapsulation most of us feel. If you are that thin then under the muscle will be more natural.
Good luck!