World Layers theory

This theory describes the structure of the world as planes or layers of finite height.

Layers
The dimensions of the world are all located in the same space over each other. If one could physically pass through bedrock or survive the void, they would be able to enter one dimension from another.

Nether portal anomaly
The 1:8 distance ratio between the Nether and the Overworld comes from a space distortion anomaly between those dimensions. The space in the Nether is denser than the space above it. Thus, relative distance between two portals in the Overworld is 8 times bigger than the portals they are linked to in the Nether. However, this doesn't cause income matter in the Nether appear bigger, as the portal gateway that connects the two dimension is also shrinked on one side, so the matter shrinks too.