Size of balls don't necessarily equal more cum ... maybe storage capacity though
. Biologically speaking, the amount of cum a man produces is equivalent to how active his seminal vesicle is (that's the gland that actually makes semen, not sperm) I make the distinction because when a guy cums that's what it mostly is ... semen Everything I've read points to that, your health/diet and your frequency of ejaculation as determinate factors in how much you ejaculate.
I would definitely volunteer to have this researched further ... for the sake of science of course