So one of your breasts is slightly smaller or larger than the other. Is this a sign of trouble?
Only if it's a new development. Technically speaking, if we were to do a 3-D CT scan of your breasts, even if you think they are identical in size, we could show you that they, in fact, aren't.
And conversely, many women will say that they've known since they were a teen that they have a difference in size. This is not unusual. Other body pairs are often slightly different sizes: arms, ears, eyes, feet, etc. Some women even have a full cup-size difference between their two breasts and they always have.
But if this asymmetry is new, if you've just noticed it and you know something has definitely changed, it's time to see the doctor. This can be a sign that something is going on that warrants further investigation. It may be a large cyst, mastitis, abscess, a blocked milk duct, or even cancer. The only way to figure this out is to see someone trained in deciphering it for you.