Well I am not a Plastic Surgeon or even an MD but I have a pretty good knowledge about women with Breast Implants, the larger sizes ones in particular.
After living in Southern California and also Central Florida for over a decade and haven taken a few trips to Vegas I have had my share and then some of experience, "hands on experience"" you might say of how breast implants especially the bigger ones look, feel, how they "hang" or do not wheter they are over the muscle or under the muscle silicone or saline, when they are "new" after a few years and after like 10 or 15 years.
I highly doubt that this current crop of "tissue expander girls" have them under the muscle.
One of the main "advantages" of going over the muscle is in general you can put it a larger implant to start with, especially if the woman is older and has had 20 plus years of gravity and/or had a kid or 2, and they blew up 3 or 4 cup sizes and then went back to the size they were to begin with which is often the case provided the woman looses all the "baby weight" and is left with that deflated/pancaked "empty sacks of skin" look.
In an extreme case rather than go the "breast lift route" which most PS's will try to talk a woman into if for no other reason that it is an extra procedure which equals extra bucks you could put in as big as a 1500 CC implant.
Under the muscle is a whole different thing. it is hard to put in much more than 600CC most PS's won't go above 400CC especially if it is a smaller stature woman.
IMO you would have a hard time getting an implant capable of exapanding to the 4 to 5 K range, just the empty implant into a pocket capable of holding a 600CC implant.
Being under the muscle means that you have a further distance to go when it comes time to do a fill, through the skin, through however much breast tissue, through the muscles which are harder to penetrate that skin or breast tissue, requiring a longer needle going further to hit that fill port built into the sidewall of the implant and a greater chance you could miss it which gives you a suddenly deflated implant.
I seriously doubt that the pectoral muscles and other muscle groups involved could even stretch to the degree we see here and if they could it would take a very LONG time years not months.
The muscle groups involved assuming they would stretch/grow over time as we have seen without tearing either the muscle or the ligaments which attach them to bone, they are more of less "vertical" attached on the top and bottom and when it comes to he horizontal are only so wide, on woman who starts and B cups probably no wider that 3 inches. They are not attached on the horizontal and also not attached to the implant so there is really nothing to keep them under stress/pressure to stretch on the horizontal axis so at some point the implant is just going to bulge out around the muscle rather than continue to stretch it, kind of the way a "beer belly" bulges over a tight belt and you get that "snowman" look or how the "pom wonderful" bottle looks rather than the big round spherical look we see.
So I think the examples we have now of woman getting very large using tissue expanders all have them over the muscle in the case of Beshine and to a lesser degree Martina Bigg we are seeing gravity take over and the only reason MM and AC have not yet had that happen is they a re not yet as big have not been really big as long and at least for now they can stay ahead of gravity by adding more and more saline but eh day will come sooner or later and I am betting on sooner when no reputable PS will add any more and they they will start heading south in a big way and if they keep them in 5 to 10 years their nipples will be below their navels.
The last pics we saw of the some of the women of the 90's when they were past their prime. Bust Dusty for example when she still had the big implants or how Kayla Kleevage looks these days.