See this pic? It's from a popular movie theater here in Tokyo (Shinjuku ward), one of the biggest theaters in the city. This is their refreshment counter. The drink sizes are labeled Small, Regular, Large.
Now first, notice that the so-called Large size would be the starter Small size in any self-respecting American megamall superplex. That goes without saying.
But now, look a little closer... notice anything about the Large size? Yep, it is sporting TWO STRAWS! In other words, the Japanese assumption is that NO ONE PERSON could possibly be a big enough pig to swill down that much of this gunk all by himself. To their way of thinking, the only way anybody would order the L size is to share tandem with their date...
So. There you have it. I'll leave the darker cultural and medical implications of this point of comparison between the two societies up to you.