Get and download a backup of your database.
Using phpmyadmin, in the view which has one row per table, select a table. One of the operations to perform upon the selected table(s) is OPTIMIZE. Maybe start on the small tables, and build up to the larger ones, maybe do them in small groups so as to not bust the quota.
Does this save you any space ?
If you can login, then install a plugin that does database optimisation, or limits revisions etc. This should let you save more space.
If this continues then you need to either organise more space or change hosting.