'Sorry the responses thus far have been slow! Occasionally, that happens around here!
This topic has come up here before, but I recall reading that, for the most part, testicle size doesn't necessarily translate into having more potency (i.e. fertility). Healthy testicle "function" is what determines sperm production and we men are subject to however that all works out for us individually. Some men are simply more fertile than others. There are men who have low sperm counts yet are NOT lacking size in their scrotums. And, vice-versa, of course.
I recall from the other thread how some guys were boasting about their having much more semen by virtue of being endowed with larger testicles. While this might seem logical, it isn't necessarily so. One has to remember that the ejaculate fluid itself originates in the seminal vesicles, prostate, and the cowper's gland-- and
not within the testicles.