Myostatin is a growth factor that prevents muscles from growing too large. It regulates muscle size from early development onwards. Therefore if myostatin is absent, muscles are able to grow much larger.

Two specific breeds of cattle have been bred to reap the benefits of its absence: Belgian Blue and Piedmontese. Selective breeding was used with the more muscled cows, to produce larger offspring.
Mice have also been genetically modified to grow larger muscles, which has led to speculation on the feasibility of myostatin inhibitors for humans. The affected mice were on average 30% heavier than normal mice, and their muscles thee to four times larger.

A German boy is known to have a mutation which blocks production of myostatin. At almost age five, he has muscles twice the size of his peers.
No doubt this could be very useful for military applications, among others.

Now we need to find one for brain development, and get governments to give newborns these drugs depending on their vocation.