Not a doctor but I think when you're getting aroused the sperm pool in a holding area in the plumbing somewhere, where they mix with prostatic fluid (?). The longer your shagging goes on before you cum the more you're going to spunk, that's what I find anyway. If it takes you longer than 4 or 5 hours to refill your balls touch.and.go and you always have a good climax after shagging, managing to empty them in one pop, then they could well be empty but it's different for every bloke. I would've said the more watery ejaculates are more prostatic fluid myself too, similar to but not quite precum (as that smells).