I, too, am skeptical of that thesis. However one plausible way of thinking about it may be that because food prices are so low (for certain foods, at least), restaurants can increase portion sizes without incurring a very high penalty. If people are attracted to higher portion sizes, then those restaurants would continue to do so, resulting in fat people. I can think of a couple of reasons that people would like larger portion sizes: a) it improves the value-proposition ($x amount of food for just $y dollars), and b) it is satisfying to feel full.
Your point about the rich is an interesting one. A conceivable explanation for this could be that higher-end restaurants tend to have a different value-proposition than lower-end ones: instead of focusing on quantity, they focus on quality.
Finally, I think the point you bring up about the summary being misleading is a good one. Reporters are generally not known for accuracy when reporting on scientific matters.
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I, too, am skeptical of that thesis. However one plausible way of thinking about it may be that because food prices are so low (for certain foods, at least), restaurants can increase portion sizes without incurring a very high penalty. If people are attracted to higher portion sizes, then those restaurants would continue to do so, resulting in fat people. I can think of a couple of reasons that people would like larger portion sizes: a) it improves the value-proposition ($x amount of food for just $y dollars), and b) it is satisfying to feel full.
Your point about the rich is an interesting one. A conceivable explanation for this could be that higher-end restaurants tend to have a different value-proposition than lower-end ones: instead of focusing on quantity, they focus on quality.
Finally, I think the point you bring up about the summary being misleading is a good one. Reporters are generally not known for accuracy when reporting on scientific matters.