Could you take a look at this post , it seem that this is a similar issue. Could you run the commands @alejandro indicated and post the result here ? If you can send two results with 24h difference that would be great, so we can see where the growing is.
Could you share in which file did you observe the growing ?.
InnoDB, the DB engine that use mysql, is not going to release disk space because of its internal working. When you delete rows in the database it is going to mark those blocks as free and will reuse them in the future. But in your case, that is growing and growing I guess that it is due the use of wp_options table.