[OnMyWay!]

Shutdown the ability to make sperm (technical name— spermatogenesis) and shrinkage should occur.

Things that reduce size:

1) high estrogen
2) alcohol (increased usage)
3) weed
4) lack of sleep
5) BPA's (aluminum)
6) cell phones
7) trauma
8) inhibiting the hypothalamus/pituitary pathway for androgen production
9) atrophy (via high E)
10) anabolic steroids

I'm sure there's more.

Lotus is correct. Once they quit producing T, they shrink quite rapidly and I have noticed that they will continue to shrink as long as they don't produce T. If you want to know if they are stil "with you" look at your ejaculate. If you still got some, then my experience has been that they still have some sort of functionality. Once mine had quick functioning, it is nearly impossible to get hard and about the only "stuff" that get produced is clear and mostly consists of pre-cum. Sorry for the bluntness, but for the science. ?