Great point Dave about the SearchItemWordPosition.
That table is related to the default DNN search box. If your site doesn't use the DNN search then you can get rid of the content from that table and from a few others too this way:
TRUNCATE TABLE {objectQualifier}SearchItemWordPosition
TRUNCATE TABLE {objectQualifier}SearchItemWord
TRUNCATE TABLE {objectQualifier}SearchWord
TRUNCATE TABLE {objectQualifier}SearchItem
But if your site does use search, you should not do that as you just created more work for the search index procedure the next time it runs as it will have to recreate the search index again.
Also, if you don't use the search at all on your site, you should not only clean up these tables above, but you should actually DISABLE the search scheduler from running all together on the site. You can do that by going to Host/Schedule and disable "Search Engine Scheduler"
Thanks,
Aderson