I am the Kat that Walks By Itself

Yes, seriously. I had recently purchased foodstuffs from http://www.usafoods.com.au, who imports them from the USA. Specifically, GrapeNuts (which I enjoy with a handful of dried currants, it's really nice). There, standing side by side, the box of GrapeNuts and the box of Sultana Bran, and it is really quite clear that the GrapeNuts box cardboard is thicker. Thick enough that I had to stand on the box to flatten it when it was empty, while the Sultana Bran box, I can flatten it with my hands, no extra force needed.

And it's not just cereal boxes, it seems to be other kinds of boxes too; cardboard packaging in general.

(goes off and does some comparisons)

The Aussie cardboard is 2-3 times the thickness of a greeting card (well, this may be not a good comparison if US greeting cards are thicker than Aussie ones, but it's all I have).
The US cardboard is about twice the thickness of the Aussie cardboard.