Dropbox tries to achieve the best possible performance by trying to be as smart as possible about what data it transfers. It only uploads changes made to the file instead of the entire file. For example, if you upload a 30-page document and only edit one page of it, Dropbox can detect and update only the page that changed. What that means to you is faster, safe and reliable syncing.
Want more detail? Here's the scoop:
Before transferring a file, we compare the new file to the previous version and only send the piece of the file that changed. This is called a "binary diff" and works on any file type. Dropbox compresses files (without any loss of data or quality) before transferring them as well. This way, you also never have to worry about Dropbox re-uploading a file or wasting bandwidth.