I had hundreds of wet dreams in my life and it wasn't because I wasn't wanking or fucking. From the age of 14 I was wanking at least once a day but still had 2 or 3 wet dreams every week.
My wife and I always had at least 3 or 4 fucks every week and usually had at least 4 every Saturday and Sunday (usually a couple at odd times during the day and a couple at night in bed. Many a time I've had a wet dream within an hour of the last fuck and as we always slept in the "spoons" position my wife usually got the full load over her arse and lower back.
I don't think it has anything to do with how full your balls are but more likely how highly sexed you are.