[Lotus]

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.