Okay, so maybe the methods I described above are not perfect. But what is? You could try embarrassing them in public. Maybe they would shrink, but then again, that may not be the best method either. My guess is that the anti-androgen answer is the correct one. They are organs, and to shrink them, I believe you would have to get them to atrophy (i.e., to stop producing).
I have been taking dutasteride for more than a year, and that did not do it. I then added green tea extract, and combined the result was clarification of ejaculate (sounds like the title of a scholarly article in the BMJ). However, no significant shrinkage has occurred. So, you may need something that actually stops production of testosterone, like spironolactone or aldactone. I do not believe that dutasteride or finasteride actually stop testosterone production. I believe they only somehow prevent or discourage testosterone from being converted into DHT. If you are joining Steve Martin in the "Let's Get Small" chorus, I think you need to bring out the big guns.