Does having an ejaculation cause your testicles to shrink?
The bulk of the fluid ejaculated comes from the seminal vesicles, not the testicles. Your testicles produce sperm and male hormones. The amount of sperm in each ejaculation is less than a droplet in size, yet that droplet will contain close to 500 million sperm cells. The rest of the fluid ejaculated is a sugar solution produced by the seminal vesicles to give the sperm energy for their long travel. Some chemicals produced by the prostrate gland that both shock the sperms into an active state and thicken the semen temporarily to make it easier to ejaculate are also added to the semen.
Even the sperm that is added to the semen at the time of arousal doesn't come directly from the testicles. The testicles produce sperm at a constant rate of about 100 million sperm a day. These sperm travel through the epididymis at the back of each testicle to ripen -- a process that takes about 4 to 6 weeks. They then travel through the Vas Deferens to await the next ejaculation. Hence, the sperm that you ejaculate was actually produced by your body over a month and a half ago.