20MB scene containing only a 2.5MB prefab?

This is the situation:
Uninty2019.2.2f1

I had a 20+MB scene that built with around 1000 instances of prefabs and very few stand alone objects that by themselves they occupied only aboput a few hundred KB.

Most of the scene is static. No lightning, no extra stuff.

Math doesn't add up, so I am trying to figure out where's the problem, but the scene only seems to go down in size by removing prefabs. I tried creating a new scene and copy all the data but the new scene had the same size.

So, I get all the scene and create a prefab out of it. I save the scene and this is the result:
Prefab with the whole scene: 2.5MB.
Scene: 19MB

How is it possible that a scene containing only one prefab instance of a 2.5MB prefab occupies 8 times more space than the prefab itself? Any hint?

 

Edit the scene file with a text editor and see what's in it.

There are also some things that can't be put into prefabs directly, like meshes. Typically those need to be saved to a separate asset. Hard to say if that's affecting you, without knowing more about the prefab itself.

Anyway, provide more information. Maybe show a screenshot of your Hierarchy view in case that helps.