This myth gets perpetuated because female bodybuilders sometimes hit the stage with flat-looking chests that are far from what we normally deem feminine. It's not the muscle that makes their breasts smaller; it's the extreme diet.
Most bodybuilders walk on stage with body fat levels that are far lower than what's considered healthy for a female. Breasts are primarily fatty tissue, so when body fat levels go way down, so do breasts. Aside from the women who have implants, most fitness and bodybuilding competitors just don't have enough body fat to keep their bras full.
Don't be afraid to press serious iron, girls.
As long as you sustain body fat levels that are well within a healthy range, you won't have an issue. According to the American Council on Exercise, women need to have at least 10-13% body fat for "basic physical and physiological health." Dipping below 10% (for ladies) may bring "underfat" health risks.
