Right, the empty install is about the same size as the demo (less than 6 GB IIRC). With the latest installer you should be able to just "update" the demo and install scenery if you want to. There are a few files that are installed with a full version that aren't in the demo (earth orbit textures for example).
If you do not install the default scenery for an area, you will only have water under your airports! Even if you add custom scenery or download Gateway airports. The default scenery includes the land textures. It's very easy to uninstall scenery, so you could swap it around all the time if you'd like & if your computer is limited in hard drive space.