500cc memory gel high profile under muscles. Is this normal? (Photo)

Thnx for pics and question.  #Yes, what you have observed is "normal" when the breast skin cannot support the weight of the implant. Some PS believe that the subpectoral position, which exerts a "plunger" effect and the relatively thin capsule have contributed to an extreme #"stretch" deformity (i.e: increased distance between the nipple-areolar complex to IMC. Options require your input and can include: 1. Do nothing, expect no improvement; 2. downsize the implants and support the implant with suture capsulorrhaphy and/or biological mesh "sling"; 3. convert to smaller volume Memory Shape +/- suture capsulorrhaphy.  Sorry.  good luck 

 
I think your concerns are appropriate; your breast implants do seem do sit quite low on your chest wall.  Given your concerns, I think you willlikely benefit from revisionary breast surgery which will likely involve capsulorraphy ( internal suture repair). This procedure serves to reconstruct the lower poles of the breasts and prevent migration of the breast implants too far inferiorly. Associated issues with positioning of nipple/areola complexes should improve with this operation.
Generally speaking, the use of acellular dermal matrix or bio synthetic mesh for patients in your situation may be helpful occasionally. This material may be helpful as a supportive matrix and help with the breast implant displacement/malposition you are experiencing. I usually recommend their use depending on the patient's anatomy ( for example quality/elasticity of the involved skin) and past surgical history.
You may find the attached link, dedicated to revisionary breast surgery (demonstrating many cases of corrective surgery for patients with "bottoming out" of breast implants) helpful to you as you learn more.
Best wishes.