I don't think sexual activity has any effect on testicle size. Mine have always been about the same and I don't recall any changes whether or not I was having a lot of sex. I've heard that some guys balls "rise up" or shrink when having sex or masturbating, and mine never really do that. Only when I'm really cold my sack will get tighter but I still don't think the testicles themselves actually ever change size. I believe testosterone may have something to do with testicle size, but not quantity of semen or sperm.
Just my 2 cents, hope it helps.