If you read the numbers on the PRP research that I posted above, then you will see something similar to what I have observed. The initial volume is from the combination of the platelets and the serum (or water) in the blood plus all the growth factors in the water that the activated platelets are starting to release.
Then over the next three days some of the the water is absorbed by the body. Then there is the growth of new tissue that continues for at least 12 weeks–bringing the volume back up to close to where it was with the initial injection.
So, volume goes up IMMEDIATELY, then goes down to various degrees but slowly over the next three days. In some people this decrease in volume is very small, in others it is a more noticeable decrease. In some, I’m seeing a growth back to where it was with the initial injection or better. In others, there is growth but not quite back to where we started. Smokers have the least amount of growth.
Again, I am speaking mostly from my experience with injecting the face. I see no reason why the same would not happen with the penis, but my experience there is less.