What causes penis to change size after prostate surgery?

This is a rather interesting topic because the word on the street varies widely from the medical consensus. Here is a stab at why that is the case:
1) The penis (actually the nerves connected to the penis) is traumatized by the surgery. Even if everything is intact, it takes a long time (multiple months, sometimes years) for the nerves to return to more normal function.
2) Since some of the nerves are right on the surface of the prostate, even the best "nerve-sparing" surgery will likely damage some of the nerves. By damage here, I don't just mean "trauma," but rather that the nerves may actually get cut, broken, or whatever, and no longer send signals (thinking of them like electric wires.)
3) When the prostate is removed, the connection between the bladder and the penis is cut and then reconnected. The difference in "length" is miniscule, so while this is true, it is not what we're all noticing.

So, my penis looked like a scared rabbit (metaphorically speaking) after the surgery, but now as I approach two years since surgery it looks a lot more like I remember it a lot more often. For a while, I had a hard time getting it to relax enough and have enough blood flow to pee standing up, even though I follow my physical therapist's advice to drink large quantities of water daily.

My friend who had first prostate cancer and now bladder cancer (over many years) says his situation is much worse.

So that's the story. It may not be physically shorter and smaller, but shorter is a reflection of the nerve status and smaller is a reflection of the blood flow, and it all interacts. Drugs like cialis and viagra work by increasing blood flow based on a chemical reaction involved, and they are often prescribed after prostatectomy to help with the recovery process. Less commonly, vacuum devices or a physical tensioning device (RestoreX) may be used to help get blood back into the penis during the months long recovery period after prostatectomy. This is because if the "smooth muscles" in the penis atrophy for too long, it can be difficult for them to recover, so all three of these strategies are designed to keep those muscles from wasting while the nerves recover.

I beg to differ on the miniscule aspect of a prostate removal's affect. If a prostate is approximately the size of a walnut, and your urethra goes through the prostate, then you lose that much urethra after a prostatectomy. The reconnection doesn't add any artificial urethra where the prostate used to be. That walnut sized difference doesn't get made up for. My surgeon said the hope is that the bladder will drop towards the penis and not that the penis will draw up towards the bladder after the reconnection, but there are no guarantees. While I am sure you are correct about the atrophy and the nerve damage, I think we have to attribute more than a miniscule amount of penis shortening to the loss of the prostate.

Retired MD here (not a urologist). Yes, my understanding is about 1.5 inches of the urethra comes out, as it travels thru the prostate. That's why men with enlarged prostate have difficulty peeing. When the urethra is sewn back together, the penis will be pulled inside a bit, as the urethra is firmly attached to the penis.

I found Australian sexologist Melissa Hadley Barrett to be very helpful in showing ways to get the penis on the road to working again when the nerves return some function. Headline: viagra drugs, vacuum pump and masturbation help during first year post-op.

Well, there you have it! Not everyone is convinced that the bladder comes down into the prior location of the prostate and the distance to the outside is within a centimeter :-). Apparently including some urologists and retired MDs :-). And thanks for adding your comments @trusam1 @tomf (And certainly including my cousin G! who preceded me in this adventure.)

Until Dennis Quaid gets back into his injectable body cam, we're left with studies and suppositions, such as this:
https://www.health.harvard.edu/blog/preserving-penis-length-after-radical-prostatectomy-20090408165

In the end, the reasons don't matter. As a general rule, the externally seen penis does shorten after surgery, and will regain some, not all of its previous length. I know that's what happened to me. Whether time, or the daily use of viagra for six months and the on-going use of a vacuum pump (which both physically stretched the penis and brings blood into the vessels) helped , I don't know.

I read about the RestoreX device being used for smooth muscle rehab a couple months after RALP and bought it and used it. Like @trusam1 I have no idea whether it made a difference, although it appeared to be helpful. I had side effects from the sildenafil/taladafil (pills) which kept me from using them and i didn't try the vacuum pump, which I only learned about being used in this way much later. It seems like the vacuum pump and the physical stretching using the restorex device might have similar benefit in terms of muscle tone?

My understanding is that the musculature in the penis itself isn't really a factor

I'm with you, @kenhorse The muscle tone is important to preserve erectile function, something that some men care about even more than penile length 🙂

My understanding was that penile stiffness is related to massive amounts of blood getting into the vessels there, and not being able to get out, due to the squeezing of their smooth muscle. So while the muscle is not actually causing stiffness itself, like when you flex a bicep, it does have a part to play. There is no skeletal muscle in the penile shaft, just the smooth muscle of the vessels. That is not under conscious control.

My urologist explained that the bladder drops during surgery for sewing the urethra back together. This makes sense to me (retired doc) as the urethra exiting the pelvis is firmly attached to the soft tissues of the perineum. I have noticed penile shortening mainly associated with ADT/Lupron therapy. After surgery, if the penis doesn't have daily/frequent erections the muscle atrophy and scars, shortening the penis. The effects of the Lupron is more associated with loss of testosterone.
That's my understanding.