Hello, and thanks for your medical question on eHealthForum.
From the history provided, a large swelling on the arm or shoulder could be a collection of the fat, or a growth from the underlying bone. Additional history regarding exact location and appearance, consistancy of the lump, etc, will be needed to know the exact cause of the swelling.
A direct face to face examination of the swelling by your treating physician as well as a Xray of the affected shoulder and arm will help to diagnosis and treatment of the swelling.
You might consider consulting your GP or a Orthopedician for additional evaluation and treatment.
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