EXPLANATION
Let's see the facts:
Regular size= 16 oz
Price = 3.36
Family size = 40 oz
Price = 7.60
The better peanut to buy is the one who gives the best weight/price rate, so dividing the weight of each butter by their price give us:
Regular size rate = 16oz/$3.36 = 4.76 oz/$
Family size rate = 40oz/$7.60 = 5.26 oz/$
So, the family size is the best choice because it give us the most quantity of peanut butter by dollar.