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.
For InnoDB tables, OPTIMIZE TABLE is mapped to ALTER TABLE, which rebuilds the table to update index statistics and free unused space in the clustered index.
That add-on doesn't alter the database. But of course there is the Rebuilding Caches stage that I imagine is the cause of this. And most likely it is for the reasons stated by ManOnDaMoon.