They do warn you about this in the plugin settings, and I can confirm it: if you plan on recording and saving stats for anything over a few days and the site gets much traffic at all, your database will grow in size very quickly.
I recently moved a client's website that had Wassup running on it for over a year, and the database had grown to almost 60MB. This site does not get very much traffic, either. I deleted the Wassup storage and the database .sql file was down to ~1MB.
It's a neat plugin for real time stats, but not practical at all for keeping stats information around so you can look it over time. It'll have to be google analytics for that.