Does a backup of your WordPress database and, or your files in wp-content/uploads and saves it in a safe location.
Switch to FTP delivery instead of using the email option. FTP is the best way to deliver large files and the email options should really only be used for small new blogs, or if you ONLY backup a small WordPress database (less than 1 Mb in size).
You need of course to have PHP compiled with ZIP. Ask your web hoting provider to compile PHP with ZIP for you or check out this blog post from Xavier Media® where it's showed how you can do it on your own server using cPanel.
You have too little memory in your server to do a backup of that many/large files. This is usually a problem for people hosting on a shared hosting account where the hosting company is using a cheap server with little memory. Select fewer files to backup, or switch to only backup the database.
Yes, this is a problem if you have large files emailed to you. Try cleaning up the old backups, or switch to FTP delivery of the backups.
Visit our support forum at http://www.xavierforum.com/php-&-cgi-scripts-f3.html