[GENERAL] pg_xlog keeps growing

I have a postgres cluster running in hot_standby. I see the pg_xlog is growing over time (may files of size 16 MB each). The replication lag is very less ~2kB, and never goes into a bad state.

I'm manually resetting this once a while using pg_resetxlog command.

Is there a way to understand why my system keeps running into this problem ?

Thanks