What I'm about to suggest is far from healthy but doesn't require cosmetic surgery either!
First off, make a note of your current bra size.
Now put on a lot of weight (as in at least double your current size).
Make another note of your bra size.
Now slowly loose the weight.
Every time you drop a clothes size, make a note of your bra size.
Keep doing that until you are back to your normal clothes size and take a final note of your bra size.
Compare your original bra size to your new one and I reckon you've got a bigger cup size, haven't you!
For example, I started off at 32DD and clothes size 10
I went up to clothes size 28 but was still a DD cup
I'm now 36GG and clothes size 18 for tops and I've still got weight to loose before I'm back to size 12 for trousers and skirts but I've gone up 4 cup sizes since I've been loosing weight!