There is no way in which to reduce the number of revisions. If you opt to keep revisions, it will keep all of them. Feel free to post that up as a suggestion there.
posts - Of course this is as large as the amount of items you have on forums. You cannot change this unless you delete content on yoru site
edit_history - You can amend how much is stored in System>Settings>Posting
log - Go to the support area of your admin CP, and visit System logs or Error logs. Each of those has settings in which you can choose how much to store
Search_index - This is the index for searching your site. You wont be able to reduce this.