XF 1.1 How to decrease database size?

I don't know whether its an issue or not - today when I logged in to my hosting account and moved to databases section I saw the size of the Database Used By XenForo Installation at http://entforo.com is of size 175 MB. Is it something unusual? Because my website doesn't have so many contents or users too. Some of the table such as jd_session_activity is of size 20 MB etc. Is it something usual or some issues?

Can anyone suggest me a way to decrease the database size?