By that logic there should also be supplements to grow taller, make your balls drop further and give you a deeper voice.
Hormones are part of the finally balanced chemical machine that is your body. They play a major part, but other parts of your physiology keep things in check.
The hormones are most important during the growing phases (from memory), before you settle into a size at the end of puberty (in which case, you stop responding to said hormones for growth).