Final answer:
In "The Binding of Isaac," the item BFFS! doubles the damage of Isaac's familiars, including increasing the damage of flies spawned by Rotten Baby.
Step-by-step explanation:
In the game "The Binding of Isaac," the item that significantly increases the size of Isaac's familiars, amplifying their damage output, is BFFS!.
This item doubles the damage of most of Isaac's familiars, making them much more effective. When applied to Rotten Baby, which spawns attack flies, BFFS! also increases the damage of the flies produced by Rotten Baby, resulting in significantly more damage per fly.