Oh, yeah, I know those. Entertaining to watch but that guy looks unhealthy lol. I'm to understand it's got something to do with injecting water or something into his sac. Odd, right?
I'm familiar with the injections of which you speak, and know there are fetishists out there who enjoy doing that.
However, I don't think this guy is one of those. His sac was large, but it wasn't HYOOGE. And in those few video clips, it was absolutely the same guy every time.
Regardless, I would think the volume of ejaculate would have more to do with the size of his prostate than the size of his balls, although this is where things get fuzzy for me. I keep thinking "size of reservoir" and possibly "size of pump", whatever those specific analogues are. The testes might produce the sperm, but I don't believe they're responsible for the bulk of the ejaculate.