I don't know. It could be.
I have big balls, and I definitely shoot a big load.
I don't know why I am even saying this, and it is probably telling more about myself than I should, but I happened to have had the oppurtunity to be a semen donor in college and grad school. One of qualifications to be a donor is that you need to have a very high sperm count and excellent sperm mobility and concentration (besides being under 30, etc, etc.) (If your semen analysis numbers are not high enough to donate, it doesn't at all mean you will not be able to have children. It probably means you are normal and fine. It's just that only a small percentage of men who begin the process actualy have high enough numbers, meet the other criteria, and are willing to follow the schedule.)
The point of this story is that I was informed that, larger testicles are correlated with a higher sperm count. Larger testicles can be a good indicator in a candidate, but it doesn't necesarily ruler out them men with smaller ones.
Interestingly, flat out semen volume is also actually a qualifier. Again, more information, than you wanted, but you actually have to produce a certain number of mls (and get it all into the receptical) to be considered. I never was told that bigger testicles produced more total semen though - which is the point of the question posted here - and as other's pointed out sperm make up only a small portion of the seminal fluid.
One thing that does seem to correlate to the volume of semen, then concentration, and count of sperm are the number of contractions during ejaculation. Contractions are different than "squirts" though - think of that you may contract a couple times during an orgasm before anything comes out, and then a quiet a few times before the last drips come out. Research seems to bear out that the greater number of contractions, the more semen, and higher the sperm count and concentration.