Actually those aren't its "balls" as people think of them. All chins have them and they're generally all the same size. And the word you're looking for is testicles I believe. My dog has a ball, he plays fetch with it. Technically what you see in the scrotal sac is the epididymis not the testes.
Usually if a male is "hanging low" it's because he's warm, if you see the sac large all the time then you are probably keeping them at the higher of the acceptable temp range or above. If the sperm gets too hot they will die, so the epididymis is able to descend into the scrotal sac to keep the sperm cooler or move up against the body to keep them warmer so they don't die from cold.