That condition is not unusual at all. In fact, it's quite common in males to have one testicle that is bigger than the other. It's just like women who have one breast that is a bit bigger than the other.
The only other thing I can tell you is this: If your testicle is swollen and it's painful to touch, then you need to see a Urologist to determine if you may have testicular cancer. Otherwise, I think there really is no problem with your testes at all.
Have you always been like this, I think if you read up about this a bit more you will find its quite normal for some men or boys as you have left no age, its a bit like womens breasts, one is always bigger than the other.
Good Luck