While lying asleep in your bed, the unspecified serum begins its journey of altering your body. Something peculiar begins to happen.
The sperm production in your body start increasing rapidly. More and more semen churning inside of your testicles. The increase in the sheer quantity of cum causes your testes to start increasing in size as well. It also seems as though your penis starts to prepare to release such a larger load as well. It starts to thicken and lengthen, growing long and wide. It reaches 7 inches, then 9, 10, 11, until it finally slows and stops at just about 12.5 inches long. Its girth has also grown to become 3.5 inches wide. Once your penis ends its enlarging process, your balls start to end theirs as well. They are have grown quite drastically and are now the roughly the size of two baseballs.
Your body remains sleeping in your bed, its new oversized genitals under the covers. Eventually, morning arrives, and you awake in your new form.