Now you've gotten your inventory counts off. There's also a (marginal) cost difference between the two size cartons. Of course, this needs to be balanced against customer satisfaction- there will be a non-zero number of customers who won't want the upsell or to buy an alternative item, and so the question is how much business would you lose vs how much money you'd make offset with the extra time and corporate headache of reconciling inventory?
Not that Sonic shouldn't do this, just throwing out some real-world considerations.