I don't think fish grow substantially overnight. I would suggest that it's one of two things: 1) when you look at a fish in a small tank at the fish store, it looks bigger than it does in a larger display tank at home. My fish seem to shrink when I move them from the 29 gallon quarantine to the 180 gallon display; and/or 2) when fish are trying to establish their dominance, they will often try to make themselves appear as large as possible by spreading their fins, etc.. So I wouldn't be surprised to see the existing fish 'displaying' for the new fish to say 'heh, this is my tank but I might let you live here if you're appropriately submissive'. Ultimately, one fish should become the larger dominant female and the second the smaller submissive male. If both fish are not already sexually mature females (which would be a problem), they should sort this out on their own.
Matt