I haven't personally had the surgery, but judging from reports from people who have, shrinkage afterward is absolutely normal.
Some of it is how the penis works as "use it or lose it"; that is, it will shrink without normal nightly erections and/or masturbation, and it will shrink with what urologists simply call "trauma", which would be getting kicked in the crotch, doing a split over a fence, or having knives hacking at your internal boy bits.
You can do penis enlargement to correct that. Pick something that's easy, like stretches or a pump, and start off gently. If you feel any internal tugging from scar tissue, don't overdo it - the scars will eventually stretch. Sometimes bits of tissue that are supposed to slide over or around each other will grow together along the incisions. Those are called "adhesions" and can be painful to correct; basically, you have to tear them loose. But they can be worked apart slowly, over months. I do have personal experience with that. Depending on which procedure your surgeon used, it might not be an issue for you, though.