When and why did American foods become supersized, according to Michael Pollan's book The Omnivore's Dilemma?

In The Omnivore’s Dilemma: A Natural History of
Four Meals, Michael Pollan explains when, how, and why he thinks
“supersizing” came into existence as part of the American
diet.

Pollan contends that in 1980, corn syrup first became
part of the recipe for Coca-Cola.  He asserts that by 1984, both Pepsi and Coke had
abandoned sugar for high-fructose corn syrup. The syrup was slightly cheaper than sugar,
and consumers seemed to have no objection to the switch.

It
was around this time, in the early 1980s, that the cola companies began to supersize the
drinks they offered, so that Americans began to consume a far greater amount of soft
drinks (and calories) than had been true before then:

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Since a soft drink’s main raw material – corn
sweetener – was now so cheap, why not get people to pay just a few pennies more for a
substantially bigger bottle? Drop the price per ounce, but sell a lot more ounces. (p.
105)

However, Pollan claims
that the true “credit” for super-sizing belongs to a man named David Wallerstein, who
had longed tried to promote sales of popcorn and soft drinks when he worked in and for
movie theaters.  He discovered that people were reluctant to buy second servings (lest
they appear gluttonous) but that they would buy “super-sizes” of
popcorn and soft drinks when they first entered the theater. Wallerstein eventually
began to work for the McDonald’s fast food chain (in the late 1960s), and, after meeting
some initial resistance there from the founder of the company, began to change the way
McDonald’s sold fast food.

It is thus to David Wallerstein,
working at McDonalds in the late 60s and thereafter, that we owe the dubious achievement
of super-sizing.