You should reduce the retention time for eventlog, it's by default 6 months, but honestly most of the data in there is not going to be useful to you at all, nevermind 6 months later. You can set it quite a bit smaller to reduce some query times.
For vlans_fdb, you have a very very large number of entries. It's possible your particular network isn't suitable to have this module enabled (a lot of very large layer 2 devices with visibility of a very large number of stations on a large number of vlans, for example a university campus).
In this instance you should disable the fdb module and truncate the table.
For eventlog, you can truncate the table now too to reduce its size quickly.
To recover the space used by mariadb (it won't free the space itself), you need to delete the tables and recreate them.
Hi Adam, we've reduced it to 1M and truncated the logs. RAM usage has returned to normal. Thanks for all your help.