I wonder that myself. I've noticed that I am downloading data when zooming in/out but the actual SN7 file size doesn't seem to get any bigger in terms of Gb.
Lead Developer of SN here. There are a bunch of reasons why we didn't deliver all that data on first download (most notably that it's now about 21GB and counting), but I promise it wasn't laziness! You have access to much more data in SN7 than was ever available in SN6, but yes it does stream down rather than being all there. (If you want it all in one go, just sign up for the beta program -it's free, fast and easy- and you can download it all in one go today.)
Note that we still deliver a LOT of data on first install, and it's only the deepest (18+ mag) USNO stars and all-sky images that currently download. The full NGC-IC (and all other) catalogs are present and accounted for in any default install.
The reason you're not seeing the application directory grow in size is because that's not where we download the data! (Both Windows and OSX forbid/discourage this.) The streamed data is downloaded (and kept "permanently") here:
\Users\<YourUserName>\AppData\Local\Simulation Curriculum\Starry Night Prefs\Sky Data\
(Note that the AppData directory on windows is often "hidden". A quick Googling can show you how to un-hide it)
/Users/<YourUserName>/Library/Application Support/Simulation Curriculum/Starry Night Prefs/Sky Data/
We're working on adding the "full data download on first install" feature and building it into our new installers too, based on comments and suggestions from users like you!
I hope that clears things up. Feel free to email me personally any time.