Semen is made in the prostate, and I'm not sure of all the factors involved in what causes us to make (and blow) more or less semen. Ball size should have little or nothing to do with it, as evidenced by when you have a vasectomy, you still blow the same load as always, but no sperm is in it. The HCG, from what I understand, fools your body into making your own testosterone again, which it does less or not at all, on cycle. That causes the testicles to go back to "normal" size, since they make your natural testosterone. Very possibly, the HCG makes the prostate more active; I don't know. I'm curious enough to go read up on it though. I have small loads, which is psychologically not so good for me, but my wife is fine with it.
Charles