It doesn’t take a ton of mental capacity, but even though I have a good education in math, I still find myself doing the heuristics of assuming that larger digits means larger number. Using fractions for comparing sizes can flip these heuristics. And I think a lot of people are like me, and also that they won’t spend a lot of time reading each item on the menu.

Where I’m from, burger sizes are just given in amount of grams, which makes it a lot easier to compare.