Yes, it's not going to make much of a difference on its own. Yes, there's a hell of a lot of other contributors to the trend of increasing rates of overweight and obesity. But people's choices and perceptions of what normal eating is are influenced by their environment, and that includes portion sizes. As @Red Herring mentioned, larger portions do lead to people consuming more mindlessly. Most people know that soda isn't good for you. But when it's everywhere and being served in absurdly large portions (some of which are called "medium" or "regular"), that makes excessive consumption seem normal. And it becomes easy to shrug and rationalize having too much.
Limiting the sizes available isn't interfering with anyone's rights either - anyone can still decide to have another.