I don't know if those sizes should be the same or not.
Maybe the file on the BIM Server is uncompressed, while the backups are compressed which can explain the huge difference.
Also, I know that the BIM Server stores data in a different format so that could be an answer to the differences in sizes.
700 MB seems like a pretty big ARCHICAD file. I think you can check whether this is the case: open the BIM Server project on your computer and then save it as a PLN on your computer, then you will see how large that would be as a Solo file.
These are just thoughts that come to my mind when thinking about this, so don't take these are cold hard facts.
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