Correction: An earlier version of this article stated that New York Mayor Michael Bloomberg’s proposal to regulate sugary-drink portion sizes would not regulate bottle size. In fact, it would regulate soft-drink cup and bottle size in food-service establishments such as restaurants, but not in grocery stores or bodegas. The text below has been updated.

TO SOME, New York Mayor Michael Bloomberg’s idea to limit soda sizes in restaurants, food carts, delis and movie theaters seems like the definition of a predatory, nanny-state mandate. Why target sugary drinks, when they are just one part of a huge problem? Why limit consumer choice in the process?

But on the other side is the astonishing fact that 36 percent of Americans are excessively overweight, treatment for which costs the country $190 billion a year. In a very real sense, obese Americans’ lifestyle choices cost everyone. And a primary driver of America’s obesity problem since the 1970s has been overconsuming sugary drinks.

Sugary drinks have been the largest single contributor to the increase in Americans’ caloric consumption over the past three decades. Soft drinks deliver roughly half the added sugar in the average American’s diet. But some Americans consume much more than others. Teenage boys, for example, now drink an average of 273 calories in sugary drinks every day. And, unlike with solid bad-for-you foods, public-health experts theorize that people don’t tend to cut back on calories elsewhere after consuming a lot of soda. That is among the reasons it’s unsurprising that consuming sugary drinks is associated with weight gain and diabetes.

So, policies aimed at discouraging overconsumption of sugary soft drinks can make sense, as long as they are part of a broader, comprehensive anti-obesity effort to improve eating and exercise habits.

Mr. Bloomberg’s approach is a particularly intrusive attempt at that. He plans to limit soda cup sizes to 16 ounces, though he would not limit the number of cups New Yorkers could buy, which means that soda fiends can still get their fix if they are determined to do so. The proposal relies on the notion that portion size can serve as a cue to change behavior, whether by indicating what a reasonable serving of soda looks like, or by simply defining when diners should feel “done” with their drinks.

That is not the first approach we would try. First, it’s full of holes, including the fact that it doesn’t regulate soft-drink bottle size in grocery stores or bodegas. New Yorkers might nevertheless bristle at the government mandate and, Cornell’s Brian Wansink suggests, save little patience for better anti-soda policies in the future, particularly if waistlines in the city don’t decline dramatically. Requiring clearer in-store warnings about health risks, for example, would be a less intrusive place to start.

Still, we are happy to see Mr. Bloomberg experimenting with serious policies to address obesity, which is more than can be said about most of America’s politicians, and we hope he succeeds. Even if Mr. Bloomberg’s plan fails to make much of a difference, it should be taken as an invitation for like-minded policymakers to try different approaches, of which there are many. The country need innovative leaders with a similar determination to take on America’s expanding waistline.