I am naturally slim and am not trying to lose weight, but I am quite interested in toning up, which I've accomplished at this point. I get cellulite when my legs and butt aren't toned, plus I like how my butt looks when I do lunges and such- much perkier and smoother.
But since I've started doing heavy lifting (squatting my own weight 3x/week, plus other lower body lifting) I've noticed my breasts getting smaller. I have not lost weight, in fact I am 10 lb heavier probably from the muscle. I still fit into the same clothing, but my breasts feel slightly smaller. Not a whole cup size, just slightly.
I'm not pear shaped, so I don't know why my breasts would go first, especially when I'm not losing weight! Why would weights on my lower body make my breasts smaller? I do gain weight first on the lower half, but I'd say I'm more of a ruler than a pear (33-24-35, 5'5", narrow shoulders/hips). Basically, I want to be healthy and toned (not hugely muscular) but keep my boobs...because I have noticed the shrinkage and I'm not alone, my mom has even commented on it!
I also have some issues with BDD so I am trying to keep this all in perspective, but it would be nice to be able to exercise and stay toned (NOT skinnier, just toned) without losing breast tissue.
You're building up muscle tissue, thus "burning" through your body fat. Breast tissue is comprised of fat and mammary glands. Ergo, more muscle = less fat (and by extension, less breast fat).
You pretty much have to accept it if you want to "tone up".
I used to be a B/C cup, but with that cup size came cellulite, doughy stomach, and soft thighs. Now I'm barely an A cup (I've stopped wearing bras), but I'm fairly solid everywhere else. Think if this way, you don't have to spend much money on bras, and you can wear halter/backless outfits without bra straps getting in the way.