MariaDB: how to reduce ibdata file size
The first thought that came to mind was &quot;Wow !!! You must have rather big transactions.&quot;
According to mysqlperformanceblog.com's Reasons for run-away main Innodb Tablespace, these are the main issues that cause ibdata1 to grow:

Lots of Transactional Changes
Very Long Transactions
Lagging Purge Thread

The undo logs inside ibdata1 will hold lots of info to support transaction isolation levels. Since you are using READ-COMMITTED, this growth can be more significant.
I have discussed this  before in the DBA StackExchange

Aug 21, 2015 : Transactional DDL workflow for MySQL
Jun 16, 2014 : MySQL Index creation failing on table is full
Apr 23, 2013 : How can Innodb ibdata1 file grows by 5X even with innodb_file_per_table set?
... and more

SUGGESTION #1
Since you are using MariaDB, please look into configuring the undo logs outside of the system tablespace. Please read the MariaDB docs on innodb_undo_directory and innodb_undo_tablespaces and go from there.
SUGGESTION #2
You may have to mysqldump are your data and load it into a new installation of MariaDB. See my posts in How do I shrink the innodb file ibdata1 without dumping all databases? for other ideas. Either way, this will take some work !!!