Here is the answer to your question. If you gain size and want to keep it. Simply quit all exercises except kegals. Do them every day, you can simply do them while you are anywhere, just flex maybe 50 times a day. Doing kegals is what will give you longer lasting erections, and better prostate health. Stop doing all PE, and you will have shorter erections, weaker orgasms and eventually lose your increase in size, in that order. That is my belief. I’ve never had any trouble with it. So yes, you will lose your gains if you completely stop PE, but how hard is it to do a few kegals every day just to keep that size? Not very when you can do them anywhere.
Kegels are simply contractions of the bulbocavernosus muscle (see this). Since the BC muscle surrounds the bulb of the penis, which is the inner part of the corpus spongiosum, contracting it will not force blood into the main erection chambers (the corpora cavernosa) nor does it have anything to do with the prostate. Doing Dr. Kegel’s exercise 50 times a day is a good idea, but it won’t do what you believe it does Tillerman.