Masturbation changes the human body by shrinking and tightening it. That shrinking and tightening of the body forces the body to move in a certain way, and also forces the body to perform movements independent of any mental desire. In other words, the body basically moves by itself.
One of the changes masturbation caused shrinkage and tightening of the body will cause is to force the individual to contract their arm, hand, and fingers in a way that looks as if the person is saying “I have a penis that is this small”, exactly like the man in this picture is doing.
As can be seen in the example image, The Sign Of The Small Penis is made by what is most frequently the masturbating arm bending upwards to place the hand by the face,
with the pinky, ring, and middle fingers curled into the palm,
and the index and thumb spread apart,
by some arbitrary distance.
The distance between the index finger and thumb will usually be determined by how much shrinkage and tightening masturbation has inflicted on the body. Emotions can also affect the index to thumb distance, usually by making it smaller because of the stress induced in the body by whatever emotion the individual is experiencing