pg_xlog on slaves has grown to 200GB

Hello,

We are using postgresql 9.5 with repmgr 3.3.2 in streaming replication
setup with 1 master and 2 slaves. I have noticed that the pg_xlog on slaves
has grown to 200GB and is still growing.

Please advise why pg_xlog is growing and not pruning itself, is there any
parameter I need to setup to accomplish this? or repmgr will control it
itself.

I am not 100% sure but feel it pg_xlog never used to grow this much or was
clearing itself, I can see there are WALs since May 2019 but not before
that. I want to understand why the WALs started accumulating since then. we
have this setup since more than a year.

Best Regards
Vikas Sharma