The effects of ejaculation are going to vary from person to person, but there are some older studies that demonstrated a couple of changes in testicle size in relation to sexual arousal.

First, as evidenced by the erection, arousal results in increased blood flow to the genitals (including the testicles and scrotum). There are varying degrees of blood flow changes during this time, and many may experience that the testicles/scrotum actually increase in size when aroused. It’s thought that this is due to increased blood flow. Once the individual orgasms and the blood slowly leaves the genitals, things return to baseline size.

Second, many individuals find that their testicles are pulled up into the groin with orgasm, so it may seem like they’re shrinking when, in fact, they’re just moving (temporarily). It’s thought that this helps with sperm release and semen production.

So to answer your question, these are a couple of reasons why it might seem that your testicle “shrinks” after masturbation (and ejaculation). However, if you ever notice permanent changes in testicle size (whether larger or smaller) it’s definitely worth a discussion with a doctor.