Your testicles do not physically get smaller. Your nutsac contracts when you're just about to cum - that's natural and happens to all of us.
That being said, it makes it only appear that they get smaller, but they're only getting closer to the body.
Kris, I suggest that you masturbate 1 to 2 times a day. The most common reason that guys get wet dreams is because they have a lot of sperm and semen saved up - wet dreams expell it. Even if there's a different reason, masturbating and using up some of your semen will reduce the chance of having a wet dream.