I've been testing this intently recently.
My test subject = Civil 3D 2014, on a stock OOTB profile, with NO network connections, including no Wifi.
My latest test involved starting a new empty (no template) drawing, and immediately saving it.
Sometimes the file is about 1MB, other times, it's about 350k. (So explain that...)
Sometimes I can close the 350k file and open it back up and save again, and it grows to the 1MB size.
The most frustrating thing is I'm trying to do this as scientifically as possible, by closely monitoring the environment, but I'm getting varied results.
It makes no sense.
I've carefully examined the 1MB drawing vs the 350k drawing - for non-graphical data and there is nothing obvious. There is some "Architectural Desktop" data embedded in the larger file, but certainly not 700k worth.
Will update if/when I find something else.