Opensource

As far as I understand, it doesn't keep copies of the different edits. The original file is saved, and the edits are serialized and stored in some form. Only one more copy is generated for the final version for preview and such. When editing again, you can add to or subtract from previous edits, and again a maximum of two files will be present. (But there would be no thumbnail, preview etc. for the original. So it isn't exactly double the storage usage.)

This is my vague recollection from what I've read in the discussions, so take it with a grain of salt. But this is close to what's happening when you're editing.