Many of these restaurants bundle 32oz drinks in a meal that is discounted. With this ban, people will no longer be able to get the 32oz drink at the discounted price. If they want 32oz's of drink, they will have to buy an additional 16oz drink at the full price. While some people may do this, common sense suggests that some won't. This means that less of these sugary beverages will be consumed.
That's the difference