How to optimize/reduce size of database?

I am using version: 3.6.10 on CentOS VPS. I am interested to know about “how to reduce the size of database”.
I have deleted all my campaigns using the Add-on but in terms of database size, it’s still same as before the deletion of my campaigns.
Now I want to go through:

  1. Optimization of Database.
  2. Reduce the size of database.