Very arguably, with 1970's manufacturing standards, and how much 32 ounces of liquid weighs, it was an engineering feat at the time. So much so that the originals looked more like a milk carton.

https://physicalculturestudy.com/2017/08/31/the-history-of-the-big-gulp/

Potts’s desperation caught the attention of Coca-Cola, who in 1976 sent representatives to the merchandise manager with a strange proposition. Coca Cola wanted to create a new 32 ounce cup for their drinks, a previously unheard of amount. The largest size at the time was 20 ounces, and even that was considered to be monstrous. Instinctively Potts refused, claiming that the Cups were “too damn big” and in Potts’s defence, he was right. The design for the 32-ounce cups was square on the bottom and resembled your average milk cartoon.

Not a feat of engineering, a feat of marketing