What might a large lump on my arm be a symptom of?

Many things: A bump must be seen and examined in person to be able to give a better idea what it is or isn't. If it's worse, changes, or not improved in the next few weeks, go ahead to a doctur.

MD exam to confirm: Odds are the large lump on your arm is a benign soft tissue mass such as a fatty tumor or muscle hernia. Nonetheless, you owe it to yourself to seek a thorough evaluation by your physician. Besides x-rays to evaluate the surrounding bony structures, an MRI detailing the makeup of the mass may also be indicated.