Today I noticed that the commentmeta table in the DB was huge, even the fact that I delete my spam daily. I saw that the commentmeta table had a size of 1,5 MB compared to the other tables with a few kb. I figured out that Akismet plugin caused that. I love that plugin cause it saved me a lot of time filtering the spam comments, but blowing my db is another thing. I have more than this blogs running with much more content. There is might be more trouble with the increase of the table.
However I executed the following query and the size of that table was only 7 kb. Woot!

DELETE FROM `wp_commentmeta` WHERE 
 `meta_key` = 'akismet_as_submitted' 
 OR `meta_key` = 'akismet_history' 
 OR `meta_key` = 'akismet_rechecking'
 OR `meta_key` = 'akismet_result' 
 OR `meta_key` = 'akismet_user' 
 OR `meta_key` = 'akismet_user_result';