Hi,
Dark patches of the skin can relate to a multitude of possible causes,like hormonal changes,sun damage,or perhaps a viral infections.Given that there are so many possible causes for dark patches around the breast, it is best for your physician or dermatologist to determine the cause.
So many women have one breast larger than the other and it's normal.
It is very rare for women to have symmetrical breasts.
Please call you doctor for a clinical breast exam.When any abnormality appears in or on the breast and you are experiencing pain,it is always advisable to seek the opinion of a medical professional.
Best wishes...
Just being larger is not uncommon. What is concerning, however, is that there is pain. It might only be an infected cyst or something else benign, but I would go see a doctor just to make sure nothing serious is going on.