In the case of WW 1 it was mainly because all of the countries involved held colonies in other areas of the globe, thus spreading the influence of the war. For example, Britain called on her colonies like New Zealand and Australia to help, thus including Pacific nations in the war. Reason 2 for WW 1 was because at the time all nations of military significance (except the US) could be located in Europe. Europe was most certainly the powerhouse of the world, where all the greatest nations could be found. This meant that when the war kicked off, not only did it concern the European nations but many smaller Asian nations like Japan who relied on European powers for trade and supplies. If even one European power at the time was to be eliminated entirely, there would be a massive power vacuum which could prove dangerous for the rest of the world as well.
WW 2 is a slightly different story. 1) Britain was no longer the greatest, largest colonial empire in the world, but still held a lot of influence in world politics. The formation of "The league of nations" (which was a primitive form of the United Nations) after WW 1 was largely because of Britain and France, and while the LoN had peaceful intentions, they had no army to back up the treaties and clauses they put in place, thus, when push came to shove, the LoN failed to stop another great war. The League included many countries from outside of Europe, which spread the effect of the war. 3) The US was directly involved in WW 2 (it was not in WW 1, they did however provide loans to France and Britain) thus spreading the fighting to the Pacific and South-East Asia. 2) China was weak, and Japan was able to take advantage of this by taking over Manchuria (a Chinese province) in 1931 providing them with the raw resources and food production required to build a large army. This was one of many short-comings of the LoN, as they did nothing to stop Japan from invading Manchuria, which made things much more difficult in the long run.
