Both the type and size of breast implant that are appropriate for you depends not only on the shape and size you wish to achieve but need to be measured against your existing breasts and your chest wall to determine what will work best to give you the result you desire. Breast
implants come in many shapes and sizes, from round to tear drop, asymmetrical and anatomic and can be inserted through a variety incision sites all of which are factors in the ultimate shape and look.
When considering which implant will give a patient the look that they want, many things must be factored in along with the patient’s desires including their physical shape e.g. current size and shape of the breast, elasticity of the breast skin, position of the nipple, and the shape and bony contour of her chest wall. For women who are very thin, generally placing silicone gel filled breast implants partly under the chest muscles is often a better choice to avoid implant show through the skin.

Before and After Breast Augmentation: D cup 400 cc Round Smooth Gel Breast Implants by Dr Hodgkinson