Just like a lot of WordPress users, I use for a client WPML to make the website multilingual. The plugin is a magical plugin that enables you to translate everything on your website. To make the translation process much easier for the user, WPML just tracks down every single added string to the database, which is not always necessary. This tracking leads to huge database usage and an exponential growth of the size of the WPML-tables.
One example of this problem is the size of the table wp_icl_strings. Because WMPL keeps track of everything, there are a lot of ghost entries, rather untranslated string, stored in the database.
Run the following query to reduce the size of the table:
DELETE FROM `wp_icl_strings` WHERE status = 0;
The query will remove untranslated strings (status = 0) from the database.
REMEMBER: BACKUP YOUR DATABASE BEFORE EXECUTING ANY QUERIES.