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Blockage of the sebaceous gland
can result in collection of sebum(oily substance)or as you have said a sweat gland
but blocked and infected that is the reason may be it has grown in size.Sebaceous cyst
can be operated easily and can be removed completely for which you need to consult a surgeon.But first rule out cancer by mammography
and it will be better to consult a surgeon first as he may give you diagnosis of sebaceous cyst by examination also.