From a recently released book by Richard McKenzie.

McKenzie did a fair amount of real-world research on the popcorn front, and his most important finding (as far as I’m concerned) is that if you’re in a cinema which gives you a choice between buying a medium bag of popcorn and a large tub of popcorn, there’s a greater-than-50% chance that the medium bag will actually contain more popcorn than the large tub.

Here is the original article.