wp_posts table extremely large

Most of the space in that table in your case is taken by the overhead. 919MB is full size, with 885MB of overhead. Overhead is caused by the storage and index write operations in the MyISAM database engine storage. You can run this query to clear overhead:

OPTIMIZE TABLE `wpwn_posts`

Or, you can convert all tables to InnoDB engine, it is the default MySQL engine now, and it is better in dealing with the overhead and just better than MyISAM engine your table is using now.