I cannot tell that posts table is large − it's very fat... Please optimize it:
Don't create revisions for all posts. At least add option for enabling or disabling this. This will reduce size of posts table in some times.
Why images and other multimedia content are stored in posts table? Please move this to separate table.
When I click on «Add post» link Wordpress creates empty record in posts table. Why?! And if I'll press Cancel button this record will leave in table. And user cannot delete it using native Wordpress tools.