Map size in bytes - decrease

Hello gentlemen and ladies!

I have a world with 7 GIGAS in the realms.

I want to decrease these GIGAS to be able to upload again (5GB).

I entered my world and broke a large building to see if the size decreases. And to my surprise it didn´t diminish.

Does anyone know how that decreases the physical size (bytes) of the map?

Thank you!