I agree it's a bit counter-intuitive. But one thing to note is that the post title/body/etc is actually stored in hive_post_data, hive_posts contains meta-data. So you need to add up those two table (and hive_permlink_data too, for that matter) to get a better idea of space consumed by posts (in which case, you get a size similar to the votes).

The other point, of course, is that there are a lot of votes cast. And there's a lot of indexes associated with voting.