That is what you have when you don't optimize and maintenance properly your Drupal projects. A 4GB database which hits hugely the server performance. Just a couple of "magic tricks" and .... 194MB.
Magic? Not at all. If you have a problem with your server, with hangs up and things going very sloooow, check the cache. sessions and watchdog tables, and clear them.
It works like a charm :-)
How important is having a database optimized. Doing some "basic" optimizations like clearing the cache, runing the Drupal cron and a couple of things more, your database can change from having 4 GB to just around 200MB... incredible? Not at all, the importance of maintenance tasks :-)
Which tasks? Mainly: