The longer you wait to masturbate, the more you will cum. It's kinda like saving up. Sometimes, a wet dream lets out all of the extra that built up.
Drinking alot of fluids - excpecially water - may increace the volume (or how much) of you cum.
It can also have to do with how horny or turned on you are. It's because all animals, humans, whatevers, are made to reproduce. If you're horny, that means cumming more is easier because all that needs to be used to reproduce.
So there's mainly
three things (in the end) that make you cum more
- How long you've waited since you last came
- How much fluid you have drank
- How horny you are