I was thinking that if she actually got this big, each breast would weigh at least 150 pounds each so most likely she would be immobile. Yes, it would be exciting to see but no one should like for somebody to be crippled in such a manner. My fetish side loves it but my rational side would not be happy. People can be complicated. 
Which always brings me back to imagining a special light weight saline substitute that weighs just 25% of saline. Imagine if you will, a foam that transforms, assumes, and fills the existing shape and becomes permanent. There any any number of products on the market that come in liquid form but expand dramatically into a foam with a fairly rigid form, so I would like to imagine a surgical foam capable to replacing the saline. Of course, both the implants and her skin would already have to be stretched to the desired size and shape, but can you imagine breasts that size weighing just 75 pounds and very possibly even less? Clearly, they would be reserved for the special girls that would love having breasts so huge that they caused big boob moments constantly.