External silicone implants for her bra ...
My wife has varied from a C cup (most anorexic) to a F cup (+100lbs.). That's more than enough breast for me. If she wants them bigger, I'd tell her to gain weight.
If she wants to be bigger while weighing less for how she looks in front of other people, I'd tell her to get some silicone inserts for her bra. I mean, I'm the only one enjoying them nude -- so if she wants bigger without the weight gain, that's the solution.
I don't want her soft breasts to harden at all -- the softness is the best part of our love making. She's in her '30s now and I love their slight sagging far more than her more perkiness in her '20s. And if I ever want them to be "more perky" as she ages 40+ (which I seriously doubt), I'll have her wear a soft bra or other see-through top (I love lingerie).
BTW, saline implants are far safer than silicone, although saline enhanced busts harden much more.