I just figured out a way to prevent the game from throwing out your underwear.
The basics are that when you buy underwear, the game will log your breast size for the purchase. If your breast size changes, the underwear won't fit anymore, but (crucially) it will still appear in your dresser drawer. You can then go out and buy new underwear for your new size. Then, if your breast size ever goes back to what you were before, your underwear will fit again.
So, basically, you'd be buying the same underwear in different sizes. It'll take some work, and won't be in the next update, but once I implement it, it should prevent underwear from being as much of a hassle as it currently is. The only reason I can do this now is because underwear is no longer weight-dependent (i.e. how fat you are no longer factors in to whether or not you can wear it).
Just thought you'd like to know.