I assume having one testicle noticeably bigger than the other is not the way it has always been; only for the last couple of years? Are you saying you can specifically feel 'tubing' or just that there's a mass? Epididymitis sounds like an inflammation, which would not be felt (I don't think) as "lots [more] tubing" (than on the other side), just lots more volume due to it being inflamed.
Another thing that can be felt (if someone has one) is a varicocele. Have you read up on those? They can sometimes be caused by doing something that pinches the blood vessels (I'm thinking of competitive cycling, but probably other sports can have an effect) and also they can happen "just because." They're basically a varicose vein. They aren't always something the doctor thinks is significant enough to address; the only reason I've heard of that a doc would do a procedure to fix a varicocele would be if a man and his wife were trying to have children, and the husband had a varicocele that was keeping the area too warm for the health of the sperm. (There's a reason the testicles hang low -- they are designed to keep the sperm a little cooler than the rest of the body.) Anyway, a doctor (especially a urologist) can usually feel one, if that's what you have.
Another thing that can sometimes (rarely) appear in the genital area is a lipoma, like 'Dr. Pimple Popper" specializes on removing from people's skin on TV. But they are rare in that area of the body.
So see a doctor, or maybe have your general practitioner refer you to see a urologist. It doesn't sound immediately urgent because you're not having pain or redness or other symptoms, but it does sound like something to get checked so you'll know what it is.
(As an aside -- I assume you're aware that you have the legal right to talk to a doctor without your mom or dad in the room? I mention it because while some kids don't give a hoot if Mom hears them talking to the doc about their testicles, others wouldn't even go to the doctor if it meant their mom would know they had a concern like this. Do go to a doctor.) Good luck!