I worked at Sonic my entire high school career, and part of my college career. Never did a route 44 button at the fountain fill a small cup. Nor did the "large" button. And the "small" button did not come close to filling a large cup.
As I stated, I can't say for other stores. I don't deny it might have only been my store, because my store manager did anything to save a penny, even if it meant screwing you over.
Also: most fast food places (where you can actually eat inside) have you fill your own cup with ice and soda, so it's not like this discussion is even relevant....
Yes, hence why I stated that the Coke itself isn't the same Coke you buy from the store.