The characteristics of an enlarged testicle, or orchitis, include swelling, tenderness, red or purple discoloration, and a heavy feeling in the swollen testicle, according to WebMD. Other indications include blood in semen, fever, nausea, painful urination or groin pain, with gradual or rapid onset.
In addition to inflammation of the body of the testicle, swelling and soreness can be located in the epididymis, the mass of small tubes adjacent to each testicle that carry sperm. An enlarged testicle can result from injury, infection or a cyst, according to Healthgrades. Less common causes are the mumps virus, testicular cancer and sexually transmitted viruses, as noted by WebMD.