The vendors will up the price of medium drinks and make a killing when people buy multiple medium drinks.
Like smoking, it will be a tight-rope walk between legislation and freedoms of choice. But smoking did take a serious hit over the past decade or two. Back in the day EVERYONE smoked. I grew up in a houseful of smokers and smoked openly when I was fifteen!!! I quit when I was 23, I am glad to report. I think most people my age do not smoke any more, and that is more than I could possibly imagine happening. It was so tightly woven into our culture.
Likewise sodas. Likewise drinks. A lot of people don't automatically offer everyone a drink when they come to the door. That was the way it was back in the day. Now there are a lot of non-drinkers in all our crowds, and it's lost its stigma to abstain.
But it all happened because of FEAR. Not legislation. It was TRUTH about the ravages of smoking that finally turned mature people off. They began to see the light. Young people are not believers in death...really. They don't really think it could happen to them. They'll figure it out.
Not be legislation. By fear. By truth.