It's not that they really shrink they simply lose fullness....balls don't disappear due to steroid use and if they do something's wrong. There's a difference between shrinking and losing fullness.
Taking more testosterone isn't going to make them any fuller, your body is simply not producing any natural test because it has no need to. Once you start producing your own test again your balls will return to their normal size. Like tbombz said HCG will help fill them out, the HCG will cause a mimicking LH effect and cause the testicles to produce some of their own test. Personally while at one time I was a fan of on cycle HCG use I'm not any longer. I find it unnecessary to begin with, you don't need any natural test if you're providing it exogenously. Further, too much HCG use for too long of a period can be damaging to future natural test production making HCG far more suitable for PCT needs if you reach a point where you're going to come off for an extended period of time.
