One of the options from Tools\Options\Catalogs is to trim the catalogs after a certain period.
What this does is keep the header on file, so that the tape is protected, but deletes all the detailed file info.
To restore from a "trimmed" tape volume, you must re-catalog first
The size isn't too bad. I've had over a Gig of catalog on one of my servers. When you plan out the purchase of a new backup server (or the server that backups will go on) just keep in mind that the catalogs can get veryu large, and spec it out accordingly