A lot depends upon what type of file you are zipping. Files such as jpegs and mpegs, dmg files without any free space on them, and other types of "compressed" file formats are already compressed such that zipping or stuffing will not produce any significant (or any) reduction in size.
Mostly the big size reductions occur for text files or data files. Zipping an mpeg, for example, is sort of like zipping a .zip archive. The main reason for zipping such files is that some servers scan for raw file formats that often carry viruses. By zipping the file it may escape the server scans.
Email is not the best method for sending large files because email tends to travel much slower than raw data via FTP. If you have very large files to transfer it would be better to find an FTP server you could mutually use or a service for that purpose like YouSendIt.com. However, unless you're willing to pay money for their use you are limited to sending files no larger than 100 MBs at a time and are restricted to a minimal bandwidth for the free accounts.
