Should I get high profile 700 cc silicone implants over or under the muscle?

Most patients undergoing a breast augmentation today go in the dual-plane/ subpectoral position.  This gives you muscle coverage over the top 3/4 of the implant while allowing it to be soft and act like a breast should.  Going on top of the muscle has a higher capsular contracture rate in all our studies, and you have a higher chance of seeing the edges of the implant in the upper pole over time.  I would discuss this with your board certified plastic surgeon and see what they have to say.  I hope this helps.

When undergoing breast augmentation there are a number of choices which need to be made: saline or silicone? Volume: Larger or smaller? Incision? However, one of the most commonly debated choices is that of implants placement: subglandular/ submammary vs. subpectoral/ submuscular? While many surgeons recommend submuscular placement there are distinct differences to each approach.
 Subglandular Augmentation (“overs”): 
Subglandular augmentation means place of the implant underneath the breast tissue but above the pectoralis muscle. Subglandular placement spares the pectoralis muscle which leads to reduced post operative pain/discomfort and no impact on muscle function post augmentation. Recovery is also fasterSubglandular augmentation can impact mammographic evaluation of the breast. However, as dedicated breast radiography has become more prevalent this has become less of an issue. Fellowship trained radiologists have become familiar with evaluating breasts post augmentation. It is also important to note that implant position does not interfere with visualization of breast tissue via contrast enhanced MRI (the most sensitive and specific study available for breast cancer detection).Studies suggest there is an increased risk of capsular contracture when implants are placed in a subglandular space.Aesthetically, implants placed superficial to the pectoralis major create a rounded, convex appearing breast profile. This effect is camouflaged, at least initially in larger breasted patients. However, as a woman ages fat atrophies and breast tissue descends. The result is a more noticeable implant specifically in the upper pole. Similarly, patients who have thin coverage superiorly are more likely to be able to perceive the implants and at higher risk of visible rippling when compared to subpectoral augmentation.Subglandular implants can also create the illusion of improved cleavage by preferential over-dissection of the central/medial pocket allowing the implants to be forced more to the midline. There is no limiting muscle as there is in a submuscular augmentation. 
Subpectoral Augmentation/Sub-muscular (“unders”): 
Subpectoral augmentation is technically a bit of a misnomer. Traditionally, subpectoral augmentation involves the release of the pecotralis major muscle from its lower attachments. This allows the muscle to “window-shade.” The upper hemisphere of the implant sits underneath the muscle (dual plane). This release contributes much of the discomfort encountered postoperatively by patients.Subpectoral implants have a lower rate of capsular contracture.Aesthetically, in contrast to submammary implants (which are prominent in the upper pole- especially in thinner patients), the pectoralis muscle both conceals the underlying implant and flattens the upper pole. This flattening effect creates a natural sloping as one proceeds from the upper portion of the implant to the lower portion.The most commonly cited drawback to sub-muscular augmentation is the animation deformity associated with contraction of the overlying muscle.In my practice, the vast majority of patients will have the implants placed submuscular for all the reasons listed above. I would caution such dramatic volume as the complication rate is much higher with oversized implants (especially when considering subglandular placement). It is important to understand...greater volume = greater weight = greater stress on supporting tissues = decreased longevity of result.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).