Skin on the breast is like skin anywhere else on the body.If you have no other symptoms,such as a breast lump, nipple inversion and discharge,breast swelling,skin changes etc..I would think that this is probably a skin condition rather than a breast problem.
A variety of skin abnormalities could be responsible for what you describe,but it's impossible to tell you over the internet what this brown spot represents,without the benefit of a medical examination.
Please consult your doctor or a dermatologist to have a look at it, particularly because the spot is growing in size.
This Forum deals only with breast cancer issues and I am sorry if I cannot be of much help concerning your condition.
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