Howdy Akis,
I would love to help you with this. It sounds like you might be suffering from an overly large amount of cached pages. You can confirm this by double checking which database table is the largest. If it is the wp_options table that is consuming much of the 600MB, it’s probably this issue.
There is a loophole in our calendar right now when you have Month View Caching enabled. We will cache any month you view in the WordPress cache. And that’s the problem. There are plenty of robots out there that visit your website, most of them like Google will not visit all of the month view pages because we have have a little flag on those pages saying they should not be indexed. But some robots will visit the pages anyways, and they can view many thousands of pages. When you have thousands of pages cached your database gets big.
We plan to introduce throttling in the future to put a maximum limit on how many pages can get cached. Until then this problem can be resolved by disabling the Month View Cache. WP will gradually empty all of the pages in the “transient” cache and your database will shrink. Or you can force it to clear the cache by using a plugin like Transient Cleaner.
Did that work? Does that all make sense?
Cheers!
– Brook