Just curious as to how this definition came about, and whether it's based on some Eurocentric construction of events or if there's some legitimate historiographical reason for this.
I listened to a recent edition of the BBC radio program 'in our time' about the Marco Pol bridge incident which started the Sino-Japanese war which was really interesting (it's available as a podcast)
I think one key aspect of the Japanese invasion of China is that they weren't trying to annexe the entire country, simply certain key parts mainly ones they felt were critical to the security of Manchuria. It also didn't involve any other countries other than China directly.
The Invasion of Poland remember brought about a declaration of war from France and Britain as well as their respective Empires, which straight away involved a large chunk of the worlds population.
According to the OED a world war is one in which the major or important nations of the world take part. It was coined a few years before WW1 and most likely comes from the German term "Weltkrieg". A war between Japan and China doesn't meet that definition.
The invasion of Poland on the other hand quickly involved a number of major or important nations such as Germany, Uk (and their Empire and Dominions) and France and it was that war that would drag basically everyone else into it.