I was steering my cart through Costco the other day, wondering whether to waddle to the aisle where they sell cashews by the quarter-ton or to the one with thousand-piece packs of chicken thighs, when an epiphany pierced the fog of my gluttony.

Actually, two epiphanies. The first? I needed to have kids, four or five or better yet a baker’s dozen. Only then could I take full advantage of the savings around me.

The second? Costco as much as anything else is why the land of the free and the home of the brave is also the trough of the tub o’ lard, our exceptionalism measurable by not only our G.D.P. but also our B.M.I. That’s body mass index, and our bodies are indeed massive.

I don’t blame Costco per se. I blame what it represents: an American obsession with size, with quantity, that manifests itself as surely in supermarkets and restaurants as it does on our highways. We drive minivans and sport utility vehicles; we rip into veritable feed bags of potato chips and wedge our steroidal Thanksgiving turkeys into refrigerators more capacious than some European cars. This doesn’t redound to our benefit.

And while the notion that we weigh too much because we buy, order and eat too much may be obvious, it’s increasingly obscured. Study after study and report upon report looks at more particular reasons for obesity and excess pounds, focusing on the edges and the aggravators of the problem instead of the flabby core. And the number and variety of these investigations, not to mention the prominent showcase we in the news media give them, create the impression that alchemy, not appetite, is our enemy, and that if we could just fine-tune our daily schedules, rejigger our protein-to-carbohydrate ratios or wallow sufficiently in fiber, all would be well.

It’s as if we’re micro-focusing on less daunting and less damning culprits to distract ourselves from the one that’s most fearsome and difficult to change, which is the sheer volume of food that many Americans are accustomed to consuming.

In The Times alone over the last six weeks, I’ve read stories or blog posts about research that explored the effect of a proper breakfast on weight loss; the implications of gut bacteria on a person’s tendency to be fat or thin; the impact that sleep deprivation can have on dietary cravings; the possible utility of strenuous exercise as an appetite suppressant; and the unhealthy food choices that a favorite sports team’s defeat can cause.

The examination of how and why we overeat is like some full-employment scheme for physicians, nutritionists, scientists and professors, who have looked at the roles of alcohol, of additives, of peer pressure, of bribes. One book that landed on my desk recently posits that pollution is making us fat and traces the parallel rise in air-conditioning and obesity.

But these experts haven’t brought us clarity. We’ve been told that snacking is good and that it’s not; that fruits and veggies will save the day and that they won’t; that more exercise means fewer pounds and that it makes no difference.

What I’ve seen with my own eyes and can intuit with common sense is more consistent and persuasive. For a few years I lived in Italy, where people are generally thinner than in America, and I learned that it’s not because they walk all that much more (one popular theory) or smoke away their hunger (another). It’s because they don’t celebrate huge servings the way that Americans do, and don’t pig out as much as a result.

The McDonald’s near my office in Rome didn’t supersize anything. I saw no roadside signs for all-you-can-eat buffets. I never encountered a “never-ending pasta bowl,” à la the Olive Garden, nor a Denny’s-style “grand slam.” Italians are content with a base hit.

Such prompts and cues make a difference. In a famous experiment eight years ago, a Cornell University professor named Brian Wansink put stale (and presumably awful-tasting) popcorn in large buckets and in really large ones. Moviegoers who got the really large ones ate much more. The vessels goaded them to.

America is lousy with such vessels: the Big Gulp, the economy pack, the party size, two-for-one pizza deals, the Whopper, the Double Whopper, the Triple Whopper, Costco in all its bloated grandeur. They’ve taught us that volume equals value and established a dangerous baseline for what we consider a sane amount of food.

And that’s a bedrock truism that mustn’t get lost amid a smorgasbord of other, wonkier insights, many helpful to a point, none erasing the importance of portion size. Yes, there are probably better and worse rhythms for daily eating; there are bad calories and good calories. The ones going into you may be sublime. Won’t matter, if you rack up enough of them.