Database-entries generated by the contact form that comes with Jetpack aren't being deleted, even though they are marked as post_status spam by Akismet in the wp_posts-table - they remain there even after optimizing and/or repairing the database table.
In wp_postmeta there are three entries for each of the entries marked as spam (_feedback_extra_fields, _feedback_akismet_values and _feedback_email). The only way to get rid of all of these entries in wp_posts and wp_postmetag seems to be to delete them manually, which is a lot of work with ten to twenty of them each day.
Please notice the discussion of this topic also here:
https://wordpress.org/support/topic/akismet-300-not-cleaning-up-data-in-database