Should I go with 625cc textured gel implants under or 800cc silicone over the muscle? (photo)

Thank you for the question and pictures.  Despite good intentions, online consultants will not be able to provide you with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
 
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you're looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your   desired goals  with your plastic surgeon. This communication will be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "porn star look" or "C or D cup" etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.  The use of computer imaging may be very helpful during this communication phase.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.
I hope this, and the attached link (dedicated to information rehgarding "XL" breast augmentation, helps.

The size of the implants are based on a combination of a patient's goals and objectives and her anatomy. In order to provide a natural looking augmentation, the surgeon needs to evaluate many factors including soft tissue coverage, skin laxity, chest wall width, breast imprint width, the shape of the breasts and relative level of constriction to ensure that the breast implants are appropriate for your body. Consult in person with 3 experienced and expert board certified plastic surgeons to understand your options.