Your catalog file is a database file which contains your backup set schedules, settings, and backup set entries. Every time you perform a backup, new information is written to this file to keep track of actions undertaken as well as data added or removed from your backup sets. A copy of your catalog is kept on your machine and, at the end of every backup, an updated copy is uploaded to our servers for redundancy.
Larger catalog files are usually seen on computer accounts which have either File and Folder or Exchange Mailbox-Level backup sets with many items in them. Since the software has to keep track of each file or mail item individually, these backup sets will result in catalog size growth. As a general rule, the larger the catalog file, the less responsive the Backup Agent becomes. If you find yourself with a large catalog (those over 1GB in size) and the backup software is running into issues such as "The process was terminated when the Backup Agent stopped responding" or the backup software hangs on actions which normally complete quickly, consider replacing File and Folder backups of data with Image backups or replacing Exchange Mailbox-Level backups with Exchange Information Store backups. These transitions are acceptable for disaster-recovery purposes, but if your client requires long-term retention (3+ months) they will need to continue using File and Folder and/or Exchange Mailbox-Level backup sets.