How does blender really save a blendfile? While working on a blendfile that is approximately 1GB and consists of many different objects, groups, materials, particle systems, etc.
When saving your file, does blender overwrite the entire 1GB blendfile or does it simply add, change or subtract certain data to the repository?
According to the Blender wiki provided by lemon:

Saving and loading complex scenes in Blender is done within seconds. Blender achieves this by saving data in memory to disk without any transformations or translations. Blender only adds file-block-headers to this data.
A file-block-header contains clues on how to interpret the data. After the data, all internally Blender structures are stored. These structures act as blue-prints when Blender loads the file.

If I read this correctly, it means that blender just performs something like a memory dump and writes that to a file and just adds some meta data to it. This is a quick way to write big chunks of data to the harddrive, but makes for a though job when re-loading the .blend file.
This is because during load you must check for changes in platform, blender version,... as described in the wiki article as well.
All in all I think the answer to you question is yes: Blender does indeed write the full 1GB file to your harddrive. It is a lot faster than writing (for instance) a 1GB Word document because blender just 'dumps' the data rather than exporting it in a document format that is fairly easy to interpret on reload.