I discovered a strange thing when I was installing a new add-on earlier to night. The database size before installation was 31.351.717 bytes. After the add-on had been installed the new database size is 30.865.285 bytes. How is that possible?
Databases have index data that take more or less space depending on your usage and if you add/delete large chunk of data.
Also, InnoDB tables in MySQL that are altered (e.g. during an add-on installation) are automatically optimized and space can be freed and retrieved at this time.