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.