WordPress wp_posts table is huge – how can I reduce it?

I have a site with a massive database (1Gb) that I’m struggling to reduce in size. It seems that the wp_options table had something like 350,000 rows; most of which were transients. I installed WP Optimize and cleared them but it only cleared about 50Mb from the size of the database.

In phpMyAdmin I can see that the wp_options table now has 23,966 rows but the data size is 947Mb. The index size is just 4.9Mb. I’ve tried optimizing the table but this doesn’t do anything to reduce the overall size of the database.

Any suggestions on what I should do now?

Thanks in advance for all help and suggestions!