There are 2 and only 2 factors involved in how much an average guy ejaculates (at least with any significance).
1. How long it's been since he last came. The male orgasm has a purpose. That purpose is to eject semen. Guys have a limited supply of semen at any given time. Sure the balls and prostate and such are constantly working to make more, but they can only make it so fast. Thus depending on how long it's been since he last orgasmed, his glands will have had more or less time to produce semen.
2. How long and how intense the build up was. Mostly how long. When a guy is aroused he produces semen at an increased rate. It's like the body's way of saying "hurry the hell up, we got an order to fill!" The longer a guy is aroused and stimulated for during sex, the more he will eventually cum.
However, the primary and most telling factor is always #1. If your guy cums a lot more than usual, it's been a while since he got off. If he barely cums any odds are he masturbated earlier that day or something.