@MikeMason-- That sounds like what I have, but yours was a tumor right?
Mine is not a tumor, it's basically dead tissue built-up from an infection I had a couple of years ago. I was very worried initially when they found a mass in my scan and I thought I had testicular cancer symptoms. Thankfully, it didn't turn out to be a tumor.
I think bacterial infections are a major cause of enlarged testicles but the treatment is easy, a course of antibiotics takes care of it. Sometimes, you can be left with a mass in the testicle though which makes it larger in size. But it doesn't pose a risk to my health so the doctor is not going to remove it.