The free market didn't make this a problem and I'm not sure why you'd think it did. The market offers larger drinks because customers want larger drinks. That and larger drinks are cheap, but make people think they're getting a better value.
Sort of true, sort of also a known fact that that food marketers try as hard as they can to create formulas that engage lower brain instincts that largely disable the brain's ability to say "nah I'm good."
Sure. But if we're going to balance that on one side of the freedom scale and government passing a law mandating what can be sold on the other, there's really no contest, right? No matter how weighted a choice appears, there's still a choice. Contrasted to not having the choice at all, it's not hard to see which course of action curtails freedom and which doesn't.
Freedom has to include the freedom to make bad choices, else it's not freedom at all. Put another way, if we empower government to take away all the choices which it decides aren't good choices, how is that different than the government simply taking all choices away from us? There's always some choice that could be quantified as "better" than another. Thus, we could follow this logic to the point of the government taking all choices away but the one it's decided is the "best choice". At that point, we have zero freedom, right?
The same rationale used to justify limiting soda purchases to a certain size can be used to justify a specific "ideal" size. Thus eliminating all choice in the matter (and think of the money we'd save if there was only one size cup that ever had to be manufactured). Similarly, we could eliminate choices of drinks to the one "ideal" drink. We could eliminate all meal choices to the one "ideal" meal. Slippery slope? Sure. But there's no reason why a government which could mandate the size of the drink you can buy cannot (or even should not) also mandate the type of drink you can buy. It's a matter of what you've empowered the government to do and why. Failing to consider what other actions can be justified with the same argument is foolish IMO. It's not a matter of whether the next step in this kind of slope will occur, but when.