Over time the amount of data stored in your local Dash database can become too great for Dash to handle effectively. Often times most of that data is no longer useful so it is just taking up space on you hard drive for no purpose.
By default Dash does not delete old data which means that the database always increases in size unless you opt to have Dash delete older data. This is easy to do but you need to opt in to have Dash delete older data.
When Dash is open, go to the Help menu and select Optimize Database.
Then check the box to "Remove meter data older than _ days" and enter the number of days of data that you would like to keep in your database.
You can also check the box to have this action performed every time the Dash software starts up.
Click continue and Dash will save and quit. When you open Dash again, it will perform the data deletion while starting up.
That's it, your database has now been shrunk in size, according to the number of days of data that you opted to keep.