A very interesting thought. This does make me wonder about the theoretical limit. If Beshine is 12k thats 24kg/53 pounds of boob. Is it realistic for Foxy to physically handle even more weight?
It depends a lot on the skill of the surgeon and how the weight is distributed throughout the body. Think about this. If a 100-pound woman steps on your foot with stiletto heels, she could actually puncture your skin because the weight is concentrated in such a small area but if she is wearing sneakers, it might irritate you but most likely not hurt you much because the weight is much more distributed in area. The same with breasts. Beshine has an internal bra which uses her ribs to help support the weight of the implants and therefore distribute her implant weight over a wider area of her torso which makes it easier to support the weight. I don't know if Foxy has such a set up but if she gets the newer and larger implants, she probably will. 
I asked Foxy about this and she said she is having trouble finding a surgeon who will insert a internal bra considering her boob weight. My feeling is that they are out there; Beshine apparently got hers when she had the custom implants placed and though we don't know, this thread seemed to guess she was 3000+cc, perhaps 3400. We look at Foxy now, she would like her boobs lifted higher (and areola aligned), the straps on her bra are digging in and unsupported, skin stretches, especially under weight. You look at Beshine and her shoulder straps do nothing beyond hold her garment up. Sol is right, sutured to bone in a higher position, the weight is far easier to deal with than hanging lower. From what I understand, Foxy would like to take care of these things, but there is no sense doing them one at a time with repeated surgeries. I can also understand the typical surgeon balking because you cannot insert a bra using traditional anchor points, especially laterally, the weight would eventually warp the ribs, but a absolute positive is that it could be better designed to fit and provide the desired degree of lift because there is no guessing what size the implants will be. A knowledgeable surgeon will place the lateral screws in the anterior shoulder, like Beshine's.
Very interesting theory in case of the internal bra. Please tell me how she managed to expand let’s say about 9k (regarding Beshine) while her internal bra is installed at 3k or 3,4k. I’m so curious how that is be done….
While not being her surgeon, I would suggest it would be logical that extra braided suture length was added to the estimate of implant size once filled. A internal bra looks nothing like a bra, but more like duel slings, usually supporting the first few inches of the implant contacting the chest. This area should change little in depth, but mostly in width as the implant expands. Foxy probably would have experienced little internal bra stress if she had gotten one at 2k because her area of attachment has changed little in size, her boobs just becoming increasingly spherical. Beshine is a different case and the evidence suggests there may have been slight displacement of the bra (sling) involving her right breast and the area of attachment is far larger, indicative of much larger implants. I don't think anyone would dispute the difference in shape between the right and left, and the lateral indentation has drawn attention for over a decade. While Beshine passed it off early as evidence her implants are still under-filled, I think the evidence supports the internal sling on that side was slipping out of place, no longer hugging the chest wall laterally, but a couple inches forward. This would also explain why the right medial suture support remains clearly visible today, because of the added stress caused by the sling being more outward on the side, the sling actually causing the indentation because it is not behind the sideboob as planned, which is why the left backboob is wider and rounder. If not for the slack intended to allow for expansion, this could never happen, but as the base of Beshine's breasts increased, this one slipped from the intended spot.
Obviously, this is all theoretical and only a CT or ultra sound would prove it correct, but Occam's razor suggests it is the most rational explanation that addresses all the irregularities. After over threes decades of assessing artificial devices under the skin in patients, I am fairly well versed in how they look, feel, and move in reference to the covering skin and I have talked about this years back on the Beshine thread. Beshine's right medial suture, or guitar string as Lacey Wildd calls hers, is and has been obvious in every picture since she was about 8k while the shoulder supports became less frequently visible as her implants expanded, elevating the crests of her breasts.
Good question though. I would suggest that Beshine found a very talented surgeon to take all this into consideration. If the next question is why didn't she get this corrected, the answer is that the sling portion of the bra is made of pig skin and it grafted itself to her own skin early on, so as her skin stretched proximal rather than distal to her chest wall, it pulled the sling away from her body. Even the best surgeon could not prevent that.