Well I think the idea of adding different portion sizes to consumable food and drink would definitely be the wrong way to go about it. On top of the charge system we already have for food, adding variations for existing foods and drinks is a huge, HUGE effort.
What would be more likely is for the hunger/thirst system to take into account the PC's size when he/she is eating or drinking, thereby giving more of an effect to smaller creatures and less of an effect to larger ones. This at least would be centralized and would snap fit quite nicely into the current system.
That said, I'm not sure this would make much sense, as a creature of a given size is going to buy and consume food and drink in portion sizes that are appropriate to the character. A full meal for a brownie is less than a snack for a half-giant. I know that brings us back to your idea, but then we run into effort/reward issues. It's one of those things where game mechanics kind of forces some compromises.