This was the first thing that struck me when I first moved here. Portions for all kinds of food from ice-cream, to coffee are huge. Why are huge portions such a common thing here?
Dealing with virtually unlimited quantities of food is a very recent problem, and Americans were the first to have this problem at scale.
Most of the rest of the world now has this problem too. If you look at overweight trends, most developed countries are about were America was 30 years ago [1].
There are other differences as well. For one, America was the first country with ubiquitous car ownership--contributing to lower population density and less walking, more time sitting, and drive through restaurants.
[1] http://economix.blogs.nytimes.com/2010/09/23/the-world-is-fa...
While other countries can now afford to overeat as easily as we can, they've been able to do so for a lot less time. Again the charts for percentage of the population that is overweight and obese back up this hypothesis--other developed countries are trailing behind the US by about 30 years. They have just had less time to make a habit out of over-consumption.
Also, most other developed countries still haven't caught up to the US in terms of affordable overeating. In PPP terms the US ranks 10 in GDP per capita, higher than every other major economy and higher than almost all of Europe by a very significant margin.
[1] http://www.feedingamerica.org/hunger-in-america/our-research...
There are also some other cultural differences. Eating out is far more common amongst similar peer groups (comparing US and UK). Additionally Americans seem less able/willing to cook, despite the amusing amount of "food porn" TV channels and shows. That also ties in with the doggie bag - the idea being that the meal served is for more than one meal, taking the difference home for eating at another time thereby avoiding having to cook later.
At restaurants, however, there is very good incentive to provide large portions. A restaurant has fixed portions that need to feed everyone from small children to gigantic football players. Without the large portions, many larger people would feel unsatisfied and not come back/give the restaurant a bad rating. Because costs are marginal it makes perfect sense that restaurants would have large portions. I've also noticed that there is a general fear of running out of food at social gatherings such as parties. Generally two to three times the amount is ordered. I'm sure this type of thinking is pretty common though this might be more of an American attitude (I don't really know).
I went to a social xmas party a few years back, and the rule was "you have to bring a dish to share". No one brings enough to just share with just one other person - everyone brings a dish/item that serves at least 4 people. We had a gathering of about 30 people and enough food to serve 150.
They changed the rule the next year (I think that was the first and only year they tried that rule, IIRC).
None of these places have anywhere near the same portion sizes.
recent studies compared obesity levels of immigrants from mexico - you see a huge spike once they're settled in the US.
the direct correlation between poverty and obesity in the US is driven by that. cheap, sugar and fat rich food with huge portions. "i didn't work my ass off to eat a fucking salad".
Or perhaps, people with childhood food insecurities and a habit of gorging what's available, when it's available, as quickly as possible will probably eat more of worse food when given the chance.
i didn't work my ass off to eat a fucking salad
... which is amusing, given that the extremely affluent classes consume well produced salads with quality ingredients as main meals, even in the face of significant choice.