Why do you think there is an expansion of any sort? For any effect a simple explanation is that some function gets reduced, and result simply looks wild.
If you try to imagine an orange on the table, you normally won't see that orange as if it really was on the table in front of you. It's a lower fidelity imagination.
There are of course good reasons for this, you wouldn't survive long in the wild if things you thought about suddenly seemed to appear in front of you.
On psychedelics, this doesn't necessarily happen either, but you can learn to visualize things in 3D as if they were real objects in front of you. Using this for anything useful takes some skill and practice, but it can be done. Some people can do this without psychedelics, but they tend to have mental health issues.
There are of course good reasons for this, you wouldn't survive long in the wild if things you thought about suddenly seemed to appear in front of you.
On psychedelics, this doesn't necessarily happen either, but you can learn to visualize things in 3D as if they were real objects in front of you. Using this for anything useful takes some skill and practice, but it can be done. Some people can do this without psychedelics, but they tend to have mental health issues.