It is not rare that individuals have somewhat unequal sizes of the chest. Sometimes, it may be more prominent but, doesn't bother people much; if anything, clothing usually hides this fact well enough. If there is no pain or other symptoms then there’s probably nothing to worry about either–the most important thing is how you feel. If an increase in size comes with pain, shortness of breath or a feeling of something being wrong then visit a doctor for some advice.