Perhaps one reason, is rather not because of the size of the conflict but rather who was involved. If you look at the people whom are involved, with the exception of the Mughal Empire, they're all European powers. Rather than say it was a world war, it was simply an extended European war that took place not just in Europe but with the colonies of the European powers.

Yes it took place everywhere but the belligerents were all European. For instance, that map includes conflicts in the Philippines and India. Yes that was a 'global' conflict in that regard but the combating sides in those was Spain, Britain, and France. In South America it was just Portugal and Spain. What can truly be considered foreign of Europe would maybe be the American theater with the Iroquois involvement but that's still, essentially, a stretch to say that makes it a "global" conflict because the Iroquois fought.

WWI is a world war because its conflict was truly worldwide in reach, there is no interpretation necessary. As opposed to the Seven Years' War, the major British colonies were now formed into quasi-independent dominions. Canada and Australia and New Zealand would fight as independent entities with their own armies and governments. More independent 'minor' nations such as Honduras and Uruguay would declare war on Germany and isn't fair to say "contributed nothing" like you mention.

Perhaps most notably would be the Japanese seizing of German colonies in China, notably the Shadong province. Japan's navy would sweep across the rest of Germany's Pacific holdings with little to no resistance as well. They would then use this position to posture against China -- tensions which were getting damn hot before international pressure forced Japan off and China joined the war as well in 1917 on the Entente side. The territorial gains by Japan and the tensions created between them and China is a direct result from the war and is a conflict born between two non-European nations.

I hope a picture is being painted. It wasn't "just" a European conflict that extended among territorial holdings that happened to be outside of Europe. Governments from all over the world on every continent got in on the war and that's why it's seen as truly global. Not because of where the people fought, but because of who fought.

I believe the reason have got to lie more in the fact that the prerequisites of a conceptual understanding of a world war was not met in the period of the seven year war, than it is a question of wether or not it was a war encompassing the world.

Concepts change, our name for one thing today does not hold the same connotations as it was then. Reinhart Koselleck really paints the image of the changes of concepts well in Preußen zwischen Reform und Revolution. Allg. Landrecht, Verwaltung u. soziale Bewegung von 1791 – 1848. (Habil. Heidelberg 1965) Stuttgart 1967.

When did WWI started to be called the world war? Didn't they used to just call it the big war?

Even though the Seven Year War took time in all parts of the world, the first world war was the first war that had a truly global character, starting with the Zimmermann-Depesche and the USA joining the war.

The wars before were mostly wars between European powers and not wars between powers from all over the world.

Adding on to that, big parts of the world were not involved, while almost all countries were involved in the Guerre.