Actually
rsync is intended for exactly that. Not only does it copy the file in chunks, when the file on the other end changes and you need to recopy it, it only copies the chunks that have changed. From the
rsync features page:
rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync. It does this by sending just the differences in the files across the link, without requiring that both sets of files are present at one of the ends of the link beforehand.
