During puberty the body naturally produces its own hormones whcih lead to an increase in the size of the penis. It seems that in later life, after puberty has finished, some men inject HGH (Human Growth Hormone) directly into the penis. Obviously the vendors of such treatments will claim "increases in penis size." Some actual users of HGH however have claimed that in the long term "a decrease in penis size" can occur. Essentially if you are unhappy with the size of your penis consult a doctor about possible treatments, do not buy drugs (particularly unlicenced ones) online or elsewhere.