Why is the largest size of Sientra round base shaped implant only a 425 cc?

Sientra makes great implants and was the first implant manufacturer in the USA to obtain FDA approval for their shaped cohesive silicone gel implants. They also sell their products only to American Board of Plastic Surgery-certified plastic surgeons in the USA, whereas the other two implant manufacturers will sell their implants to any licensed physician who wishes to perform breast augmentation, regardless of specialty training. AND, Sientra offers a 2-year capsular contracture "warranty." This is not a "new" company, as they have been manufacturing and selling implants worldwide for many years, and were the first company to be allowed to sell implants in the USA since the FDA restrictions of 1992 caused all but Mentor and McGhan (now Allergan) to go bankrupt or cease selling implants. So certainly a solid choice for implants, as are the other two companies' products.But, I question your central premise that a "natural" look is achieved by a shaped implant more commonly or better than by a smooth round cohesive gel implant by Sientra or either of the other two implant manufacturers.
Since teardrop-shaped implants must have proper
positioning and must stay in that position (sloped part up, rounded part
down) to take advantage of their innate "natural" shape, by necessity they are
textured so that the body's tissues can adhere to and maintain the
proper position of the implants. Thus, teardrop implants
cannot and do not move very much at all as the patient changes from a standing to a
reclining position. Also, the teardrop implant remains teardrop-shaped
when the patient is lying down, whereas the natural breast flattens out
and assumes a round appearance. Shaped (form-stable) implants are
also more costly than round smooth implants. The smooth round implant
does just what a natural breast does: it is teardrop-shaped when the
patient is upright, and flattens and drops slightly to the side when the
patient reclines. Thus, the round implant is actually more "anatomic" than the teardrop, “gummy-bear,” or so-called anatomic implant.Some plastic surgeons believe that the “anatomic” or teardrop-shaped implant has a
more natural look than the round implant, and much advertising and
marketing has been used to promote “anatomic” implants to the public.
These implants are perfectly good devices, but they are more expensive than
the smooth, round implant, and whether or not it is truly more
"anatomic" is questionable, and has actually been studied. In this study, actual
women with smooth, round implants were compared to women with textured,
teardrop-shaped implants. Mammograms were performed in upright and
supine (lying-down) positions, and digital tracings computerized and
compared. Both the round and teardrop-shaped implants had a sloped,
teardrop shape when the women were upright. In fact, in upright
patients, teardrop and round implants were indistinguishable on X-ray, because the soft, cohesive silicone gel round smooth implants became teardrop shaped themselves.
But, when a woman is lying down, the natural non-augmented breast
flattens, drops slightly to the side of the chest, and assumes a round shape, as
does the round smooth implant. The teardrop-shaped implant stays
teardrop-shaped and does not move at all to the side in the lying-down position, which makes this actually LESS anatomic than the round implant! I just thought you should know that before you are "sold" a (usually) higher-cost pair of implants for reasons that are not necessarily valid. Best wishes! Dr. Tholen

Yes, Sientra's upper limit on volume for shaped high strength silicone implants maxes out at 425ccs - they have said larger implant volumes will become available. However, the projection of a round based shaped implants ( the one's giving more projection)  equals that of a round implant of a high profile projection of 600ccs or a moderate plus profile round implant of appx 800ccs. Remember that the projection and volume is centered in the lower aspect of the breast where it is supposed to be, instead of using extra volume creating a more rounded shape and fuller upper pole of the breast. You get more implant projection and shape of the breast with lesser volume of ccs that creates more and better shaped breast from the overall implant volume placed in compared to round shaped implants.