When I used to do R&D for Moonstone back in the day, I would use whatever bags they would ship me. Consequently I got to demo many shapes and sizes.
I found bags that were just a bit short or even "the correct" length to be a bit less comfortable around the head and shoulders. For 2 oz. I'd get the longer one and take a handful or two less gorp!
And like JL said, it gives a bit of room for warming stuff and to store what extra food won't fit in your bear can; like the bacon for breakfast!. (Just kidding folks! though it has been done)