The 32 oz "Big Gulp" was introduced by 7-11 in 1976 in Southern California:

https://en.wikipedia.org/wiki/Big_Gulp

Even bigger sizes arrived in the 1980s:

Called Big Gulp because it was initially the largest such drink available at any retailer, 7-Eleven eventually introduced larger sizes. In 1986, they introduced the 44-US-fluid-ounce (1,300 ml) Super Big Gulp, followed by the 64-US-fluid-ounce (1,900 ml) Double Gulp in 1989 (later reduced to 50-US-fluid-ounce (1,500 ml))...

They were more absurd in the 90's. You'd have the gas station 64oz reusable cups you could use to do unlimited refills for a dollar.

"Super Size" was a 90's thing as well, if I recall. It was basically an arms race of which company could provide the most ridiculously large portions of a cheap product like soda or fries.

Then the PR swung in the 00's due to the obesity epidemic and such stuff was silently rolled back. Likely for both PR and cost reasons. The 32oz drink stuck around though pretty much everywhere.

Soda is super cheap to make though. It's usually just tap water with some processed sugar and some artificial flavourings.

It's an easy thing to promote more of to divert attention while reducing something else that actually costs money to make.

Or why portions of fries go up when burgers go down in size. Cheapest food item.

Oh yes, the 1990s was the height of massive fast food drinks. Wendy's had 32 oz "biggie" drinks and a 40+ oz "great biggie". McDonalds had "super size" tho I don't recall its volume. Most of those got renamed by the mid 00's and eventually downsized.