I think excessive size and firmness are linked. They can't use the softer feeling implants on girls going up in size significantly, since the implant would basically pancake out. I suppose they could probably use a firm implant to stretch things out and then replace it with a softer one. Or somehow develop one that loosens up over time.
A former fave got hers done recently, and they're awful. She had nicely shaped small B's that were admittedly a little on the small size. But she got giant ones that I'd guess were on the borderline of C's and D's. The nipple placement is a little off and they're very firm. I think they had to be firm to retain any sort of shape after being squeezed into her tiny frame. The bigger problem however was that she took off work for the aftercare, which was good in that she doesn't have scaring or anything. But, its bad because she put on considerable weight that doesn't seem to be coming off. So, she went from being a very cute, slim and firm girl with well shaped but smaller than ideal tits to a kinda pudgy girl with tits that are hard and don't really flow with her body shape.
Had she gone with a more modest increase, maybe to a small to mid C cup they likely would have been able to use a more natural feeling implant and her recovery time probably would have been shorter, reducing the weight gain. The result potentially being a small framed girl with a little meat on her bones and a proportionate rack, which is something most would have approved of.