mysql.gtid_executed table growing huge on read only slave

I have a setup with one master and one slave running on Percona 5.7.12
using GTID, the slave is set to super_read_only to prevent updates.
Both, master and slave have binary logging enabled with relay_log_info_repository and master_info_repository set to table.
Replication is working fine but I run into the following problem:

The mysql.gtid_executed table on the slave is growing very big (>1.5GB in one day).
Reading the MySQL 5.7 documentation I read that the table will only be compressed on a bin-log rotation.
Last rotation is two days ago, when i restarted the server.
So I assume that the bin log will never be rotated, as there are no writes beside the replication on the slave, and i fear the mysql.gtid_executed table will grow forever ?

I hope I am missing something here and would be glad to get some advice.