Why is it that one of my testicles is significantly larger than the other?

Not normal: But need not be a problem unless smaller testis is much higher than the other, have had mumps in the past, or have a large varicocele or hernia on small side. Larger side might have a hydrocele. Testes can grow at different rates during puberty & smaller one may or may not catch up in time & both testes can otherwise be normal. Thus although not normal maybe ok. Scrotal ultrasound would clarify.