The comparison with the dungeons present in Diablo is wrong. The 8 levels or grades we are talking about are all overground and each one is big as a region.
To better understand how the world is, take a look at this picture:

The city you are visiting in the game now is in the 1st level, that in the image is the first green circle to the top. To give you an idea, the first circle is about 100 times the area you can visit now in game.
The distance between one level and the next one is in terms of kilometers, so they represent really different regions/cultures.
The Stone Labirynths are another additional regions that can accessed by entering one of the \"walls\" present in the 1st level. They extend all around the level and go up.