Hello and welcome to Tom's Hardware Forums.
When you save an existing Word document within your own computer, the new data are added to the old - you may have noticed that a temporary version of the original is created a soon as you open the document and that. That also applies if you're working across a network. However, when you copy a flie from one system to another, the entire file is transferred and overwrites (with a warning) a file with the same name if one exists in the target system.
How are you transferring data - copy and paste, cut and paste or SaveAs?