I'm male 17 why has one of my testicle grown pea sized?

Testicular atrophy: May have had testis torsion before birth or in infancy if always very small. Could be result of later torsion if testes used to be same size. U would have had an episode of severe testicular pain & swelling which lasted for a few days & resulted in demise of your testicle. Should see a urologist if this was the case because other testis may be at risk & require simple procedure to secure