Postgres Streaming Replication Disk Usage Discrepancy - postgresql

Question

Just a quick question - apologies if its been asked before, I couldn't find it.
We are using asynchronous streaming replication with postgres and have noticed that the disk usage for the database can vary between the master and the replica, even though the databases appear to be synchronising correctly.
At the moment the discrepancy is quite small, but it has on occasion been in the region of several GB.
At present, this is the sync status:
Master:
master=# SELECT pg_current_xlog_location();
pg_current_xlog_location
--------------------------
35C/F142C98
(1 row)
Slave:
slave=# select pg_last_xlog_receive_location();
pg_last_xlog_receive_location
-------------------------------
35C/F142C98
(1 row)
The disk usage is as follows. Again, I realise the discrepancy is currently quite small (~1.5GiB), but yesterday it was several GB.
Master
-bash-4.1$ df -m /var/lib/pgsql/
Filesystem 1M-blocks Used Available Use% Mounted on
/dev/mapper/lv_pgsql
401158 302898 98261 76% /var/lib/pgsql
Slave:
Filesystem 1M-blocks Used Available Use% Mounted on
/dev/mapper/lv_pgsql
401158 301263 99895 76% /var/lib/pgsql
I should clarify that the archive command is set to archive to a different partition on the master. I guess what i'm asking is:
- Is the current disk usage discrepancy normal?
- How can it be explained?
- How much of a discrepancy should there be before I get worried?
Thanks in advance for any help.
edit - I'm specifically interested in the discrepancy within the "data/base/*" directories which house the actual db content, as follows:
Master:
7 /var/lib/pgsql/master/9.3/data/base/1
7 /var/lib/pgsql/master/9.3/data/base/12891
7 /var/lib/pgsql/master/9.3/data/base/12896
57904 /var/lib/pgsql/master/9.3/data/base/16385
180 /var/lib/pgsql/master/9.3/data/base/16387
11 /var/lib/pgsql/master/9.3/data/base/16389
203588 /var/lib/pgsql/master/9.3/data/base/48448446
7 /var/lib/pgsql/master/9.3/data/base/534138292
1 /var/lib/pgsql/master/9.3/data/base/pgsql_tmp
Slave:
7 /var/lib/pgsql/slave/9.3/data/base/1
7 /var/lib/pgsql/slave/9.3/data/base/12891
7 /var/lib/pgsql/slave/9.3/data/base/12896
57634 /var/lib/pgsql/slave/9.3/data/base/16385
180 /var/lib/pgsql/slave/9.3/data/base/16387
10 /var/lib/pgsql/slave/9.3/data/base/16389
202945 /var/lib/pgsql/slave/9.3/data/base/48448446
7 /var/lib/pgsql/slave/9.3/data/base/534138292
1 /var/lib/pgsql/slave/9.3/data/base/pgsql_tmp