The answer is quite simply - NO. Developing your pectorals(chest muscles) won't make your breasts smaller. Many of you would have heard this tale countless number of times, but let me tell you now that this is yet another myth about women's weight training/female bodybuilding that just does not seem to go away!

Yes, strength training has an overall toning effect, however - if you're dieting and you lose a significant amount of body fat, THEN you will reduce breast size (but of course, this decrease will also be proportionate to the fat loss from your entire body). So if you lose weight and end up with smaller breasts.. its because thats how you truly were meant to look like.. and there's nothing wrong with that. So blame the shrinkage on your diet/weightloss routine, dont blame the exercise.

Muscle and fat are very different types of tissue and we cannot turn one into the other, and womens breasts are mostly fatty tissue. Your breasts won't actually get bigger, but what is actually happening when you perform the right chest exercises is that you would tone the muscle underneath the breast tissue, and this will cause your chest to 'appear' to be bigger. Therefore, it is impossible to actually increase breast size through weight training but what you're able to do is to only create an 'appearance' of bigger breasts. Isn't that good enough?

Truth is, if you go below 12% body fat (which you shouldnt be unless you're a competing female bodybuilder, etc), your breast size WILL decrease.. so don't be surprised when that happens. If its still bigger breasts you want.. then the only solution for you is to either gain fat or get breast implants.