I honestly don’t know if I can get a quadruple Whopper in my area; I’ve never eaten more than a double, and that was on the… extravagant side of things.
My personal theory, which is basically what I understand of history seasoned with a bit of business school, is that until the 1950s/1960s, fast food wasn’t even a thing. There were a small handful of local takeout type joints, but they were competing more with home cooks than anything else.
Once national and regional fast food chains became commonplace, they had to have a way to compete with each other- what was going to distinguish McDonald’s from Whataburger from Burger King, from Jack in the Box? Menu offerings was/is one way they do it, speed is another, but the third leg of that stool ended up being size.
It started relatively slow, I’m sure- slightly larger burgers, an extra size option for fries and drinks, etc… But at some point in the mid-late 1980s, after a frenzied round of diverse menu options (Arch Deluxe, McDLT, BK International chicken sandwiches, various sorts of chicken nuggets, Frings, Bell Beefers, etc…) size became the big deal- it seemed like suddenly the restaurant chains were about double quarter pounders, triple whoppers, super-sized fries and drinks, etc…
In parallel to this, you had the rise of the fast casual restaurants like Chili’s, TGI Friday’s, etc… and of the counter-service places that were in between them and fast food- the Taco Cabanas, Five Guys, Chipotle, Boston Market, etc… They basically had the same issue- how to compete? When your competitor decides their “standard” burger is a 1/3 lb patty, you sort of have to have an equivalent, or look like you’re providing a wimpier, less valuable burger by comparison.
From there, it’s just been a virtuous circle (of sorts) where the various restaurants are trying to out compete each other based on portion size & “bang for your buck”.