Ever had a 12 oz can of Coca-Cola and thought, “Gee, that was refreshing, but I could have used an extra 1/2 oz”?
If you said yes, they’ve got the product for you: a 12.5 oz bottle of Coke.
The same gas station with this sign also advertised a 16 oz bottle of Pepsi. Presumably that’s for people who need an extra four ounces, but don’t want to spring a standard 20 oz bottle.