Table cache_similarterms very big

I am going to assume you are using the latest version 6.x-1.18 since 6.x-1.9 didn't have the cache.

Are you running the cron process? At least every hour at a minimum. Preferable more frequently I run mine ever 5 min. The cache table should be getting cleared by the standard Drupal purge process. Based on how I understand the code it will be 1 hour Plus 1 seconds after the 1st person views the page that gets cached.

I just checked my DB. I use 1 Day caching and my DB is at 6.6 Megs. Things that will help reduce the size of the DB. 1) Increase the frequency of the cron process that will purge them faster once they expire. 2) Decrease the time the cache sits arounds. 3) Under settings there is a option that will expire the entire cache.

Thanks
Robert