Do guys with big balls cum more either number of spirts of total volume?
The vast majority of what comes out when a man cums does not come from the testicles. It has been shown that men with larger testicles (and animals as well) tend to have more sperm per ejaculate (thus have the potential to be more "potent", more sperm in each ejaculate); but that also does not always hold true; but the amount they ejaculate is not determined by testicle size. So a man with normal to even smaller testicles can "cum" in as large (if not larger) volumes than a man who has large testicles. Also remember that a man still cums after a vasectomy, where NO sperm is added to the ejaculate.
A similar thing is true with breasts and producing milk. One MIGHT think that a woman with larger breasts by default produce more milk... hey, they have bigger breasts, right? Um... no. It is not quite that simple.